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Right. I imagine you've just read the introductory page for this project? In this section I get down and dirty with some mathematical calculations based upon advice given at a number of Internet resources during my research. Don't worry. Skip to the pictures in the Build Section if you just want to see how I built it, but you might find this design section useful to help your understanding of the processes involved, and what is generally considered necessary in the design of a biological filter.

Jims Pond Blog

To get the latest news on my ponding bio-filter and venturi experiments why not visit Jim's Pond Blog. You can subscribe to my blog's RSS Feed powered by Feedburner to ensure you get the latest updates. It will work with most Atom and RSS 2.0 compatible news reader software, such as Bloglines, Desktop Sidebar, NewsGator, MyYahoo, etc.

This page covers the following topics (click to jump to them):-


For great fish-keeping communities visit my favourite forums at:-

They are very friendly and knowledgeable groups of people who will make you feel very welcome. There is tons of discussion going on about fish of all kinds, problems whether relating to the health of your fish or the state of your pond, and advice on filters, pumps and anything else you can think of!


Mike Bentleys pond page has a very comprehensive ponding links page with some excellent homemade DIY pond bio filter related links showing a wide ranging variety of ideas and experiments.

I highly recommend Bradshaws Direct (UK) for all your ponding supplies:-

Sick of SPAM? This is what I use every day - It's very good.....

Read my review about Mailwasher Pro Anti Spam Software

Overview and Design Points

If you're done at the Skippy site after reading the previous page, you now have a pretty good idea of whats involved in building their Skippy bio-filter, and how it all works. This picture is of their basic Skippy filter "in the raw". Hmmm. I think "functional" is the word! Well, it's not so bad after a bit of hiding!

A Skippy bio-filter, built using a "Rubbermaid" water tank. Note the large flange outlet.
A Skippy Bio-Filter
Placing plants actually in the top of the bio-filter is beneficial to its overall efficiency
Where did it go?

I like the whole idea behind the Skippy design. However I had some ideas of my own:-

  • My pond is not massive (about 400 gallons), so I didn't think I needed a tank as large (or ugly) as a "Rubbermaid" used for the Skippy filter. Please remember that I am not a professional Koi-keeper! I don't need a massive filtration system, I just want to improve the water quality for my small pond.
  • Generally a Skippy filter is situated at the top of, and is effectively part of a waterfall, with its large flange outlet (see above picture), the water literally flows out and down the waterfall (with rubber sheeting around the flange to prevent water leaking behind any rocks which form the waterfall). To accomodate this size of tank into my already built rockery simply was not practical, and I wanted a design with flexible pipe to put the water where I wanted it - in the back of my terracotta urn.
  • I particularly liked their "vortex" design, where the pipework creates a swirling motion in the base of the filter, but a "Rubbermaid" is oblong. Using a cylindrical tank would be better to maintain a smooth swirl in the bottom.
  • I wondered whether I might find something that looks nicer, and could "blend" into the garden better.
  • Rather than having a drain outlet in the side, I thought that a drain exiting vertically down out of the base would help remove filtered solids more efficiently. A Rubbermaid tank has a small outlet in the side, near the base, which is fine for draining just water, but not great for getting gunky muck out of a bio-filter!
  • After reading up on "venturis", I wondered whether a venturi dedicated to aerating the filter would improve oxygenation of the aerobic bacteria in the bio-filter.


Well, here is a picture of my working bio-filter in position (in 2004 before the venturi was fitted). I find this size is sufficient for my 400 gallon pond.

I suppose a serious koi-keeper might laugh his socks off! Thats fair enough, and I admit I am a bit eccentric but I like to try out my own ideas. In time I might paint the pipework a terracotta colour so its not so garish, and grow some more plants both inside it (to help filtration), and around it.

I might also add at this point that by building this filter myself I did not save money! I think I must have spent around £100 by the time I had finished experimenting and getting all the parts together. If I had known exactly all the parts I needed right from the start it would have been cheaper, more like £70.

I could have bought an off-the-shelf prefabricated product of similar size for £50, but I don't think I would have enjoyed the challenge as much as doing it myself!

A big pot with some pipes to make the water go upward and round in circles, filled with green scouring pads, and later a venturi.
My original 2004 version of a DIY bio-filter

A Quick Recap

Before we continue lets recap on what we're trying to achieve;

Biological Filtration and the Nitrification Cycle

A biological filter is quite simply the heart of a koi pond. It is not essential in small fish ponds, but the more fish you stock (especially koi), the larger they get and the more they eat, so the need for a bio-filter becomes greater. The pond gets to a point where it needs a "sewage farm". It's purpose is to convert the waste matter produced by the koi from harmful ammonia into less toxic waste.

It is less important to remove solids particles from water than it is to process nitrogen, so if there is to be a compromise between mechanical and biological, err on the side of biological.

In other words, it is much better to allow particles below a certain size to escape back into the pond, while converting a great deal of ammonia to nitrate, than it is to catch every little thing down to a micron or less which in the process would slow the water down to the point where the bacteria have a hard time living (because they're not getting enough oxygen).

The bacteria that convert ammonia to nitrate for us are among a class of bacteria that you may have heard of before. They are the so-called, “nitrogen fixing” bacteria. This means that they take nitrogen that is unavailable to plants in its ammoniacal form, and make it available to plants in an oxidized form.

These are the same bacteria that live among the roots of leguminous plants. Without these beneficial bacteria, life as we know it would cease. So be kind to your bacteria. What they need to survive is a large surface area, chemically inert medium and a ready supply of fresh water. They depend upon dissolved oxygen in the water to live and to do their job. As soon as the water flow is stopped, the oxygen in the filter becomes finite, and eventually gets used up. The ultimate result is that the bacteria die, and you have to start over. Also the bio-filter will benefit from a boost of new fresh bacteria on a regular basis, say monthly, to replace any that die-off naturally thus ensuring the process in the bio-filter continues un-interrupted.

Some people use both mechanical and biological filtration, and some of the new commercially manufactured filters available do this. In the first instance, many people are tempted to install a canister filter (pre-filter) ahead of the biological filter, thinking that they will extend the life of the media used in the bio-filter by catching all of the particles that would eventually clog it up. They are correct, of course, but they wind up being slaves to the canister filters in an effort not to destroy their pumps, which have to work too hard to push water through the pre-filter canister.

It is better to merely use the bio-filter and to maintain it well.

Putting the canister after the bio-filter is a good idea, but the same problem arises, just less often.

Several types of bacteria are currently available to you from your local pond centre stockist, each with its own specialty. It is important to start out with a good supply of beneficial bacteria, especially for the initial start-up dose, so that you minimize the green water stage of pond establishment.

There are 2 types of bacterial species that colonise the biological filter media. Nitrosomonas sp. bacteria which oxidize ammonia to nitrite, and Nitrobacter bacteria convert nitrite to nitrate.


Ammonia (NH3) is produced by fish (and particularly koi because they are fat greedy chaps!), as part of their normal metabolic function and is excreted from the gills. The amount of ammonia produced is directly related to the amount of food they eat. Approximately 3-4% of normal 30-40% protein level koi food will be excreted as ammonia, i.e. for every 100grams of food 3-4grams (3000-4000mg) of ammonia is produced.

Koi exposed to unacceptable levels of ammonia risk damage to gills, eyes, fins and skin which can result in them being susceptible to secondary bacterial infection.

Using standard drop type tests kits any ammonia reading is considered unacceptable and remedial action should be taken.


Ammonia is oxidized by the Nitrosomonas sp. bacteria in the filter to produce nitrite (NO2). Whilst it is not considered as dangerous as ammonia it can still do serious damage to your fish. High levels of nitrite are likely to stress your koi leaving them susceptible to secondary infection. As with ammonia, target levels should be that nitrite is undetectable.

Before the fish pond filter can efficiently remove ammonia and nitrite from the fish pond water, it must first become fully colonized with nitrifying bacteria. This can take some time and is a process known as fish pond filter "maturation". Each time a fish is put in the fish pond it will add to the total amount of ammonia being produced. The ammonia level in the fish pond will therefore increase slightly. Because there is more ammonia for the bacteria to utilize, they start to multiply until there are enough to use all of the ammonia being produced inside the fish pond. The ammonia level in your fish pond will then fall back to zero.


As the ammonia level falls, the amount of nitrite produced by the bacteria in the fish pond filter will start to increase. Therefore, the level of nitrite in the fish pond will rise. The increasing nitrite level means that the bacteria that break it down can start to multiply in the fish pond filter until, as with the ammonia, there are enough to use up all the nitrite that is being produced. The nitrite level within the fish pond can then fall to zero. As this occurs, the nitrate level increases.

Conversion of nitrite to nitrate (NO3) is the final stage of the nitrification process. There is debate as to the possible problems that elevated levels of nitrate may cause. Indeed some koi keepers have high Nitrate and it causes no problem at all. High nitrate may also attribute to green water (phytoplankton) and blanketweed growth however the two do not always go hand in hand. The green water problem can get worst when you clean the biofilter and make water change outs, due to the reduction in bacteria - these are times when you should add some fresh bacteria to boost the level.

The bacteria also produces a certain phytoplankton-killing enzyme. As algae starts to grow in the bio-filter, or on the walls of the pond, the bacteria loves to feed on this algae, and as it does so it releases the enzyme (like an antibiotic) into the water. (Fascinating source article can be found here: http://www.vcnet.com/koi_net/GRENH2O.html).

Green water is a pain for many reasons. Ultra Violet Clarifier lights will kill single cell phytoplankton algae that cause green water, and when dead they clump together and can be removed by the filter. However there is sometimes a concern expressed that passing water through the UVC also kills beneficial bacteria. Note that a UVC does not get rid of blanketweed.

The Skippy site teaches us that we should try to achieve "balance" in the pond - don't fight mother nature. By use of the bio-filter and other larger plant forms you starve the water of Nitrate, so that the algae has no food, and is therefore unable to grow, while at the same time the bacteria create the enzyme which kills the phytoplankton. Its a double-edged sword in this battle.

So in summary:-

  • Nitrificating bacteria reproduce slowly.
  • It can take between 1 and 2 months for a biological filter to start to produce enough bacteria to sustain a pond system.
  • To avoid dangerously high levels of toxic waste levels in a pond, do not add too many fish at one time.
  • Do not switch the filter system off for an extended length of time. Nitrificating bacteria will die after only a few hours without oxygen, leaving behind anaerobic bacteria.
  • Anaerobic bacteria is toxic to fish and is a common cause of disease in fish.
  • A dirty pond/filtration system can harbor anaerobic bacteria and koi diseases such as outbreaks of parasite infestation and bacterial problems may occur.

Click here for more information about Bacteria Cultures and products suitable for Bio-Filters.

Some Theory and Maths


During my research I came across a very complete and excellent resource by Terry Cusick,Third Alternate AFKAPS Rep
and Certified AKCA Koi Health Advisor at FishDoc.co.uk to whom I extend my thanks, and from which a number of extracts have been used to compile this section of my project. I strongly recommend reading the complete article since the site provides a very thorough explanation of the considerations when designing filtration systems for ponds to ensure healthy fish.


Humans convert ammonia waste from the body into urine, whereas fish simply excrete it continuously from their gills into the surrounding water. Normally in a river or the sea it is diluted by thousands of gallons of water to render it harmless. But nobody told Mother Nature about koi-keepers and their ponds, where ammonia can build up to a dangerous level due to the large number of fish in a small volume of water.

A koi pond has to deal with two types of pollution; solids waste and dissolved waste from solids. Therefore it is essential to remove the solid wastes from the water before they have a chance to dissolve. If we can do this we gain; better water quality, fewer dissolved pollutants and ultimately less fish health problems.

Once the solid wastes have been collected, it is important that they are flushed out of the system regularly, before they get the chance to decompose. In summertime this could be as often as twice a day. This means that any settlement chamber incorporated into the filter design will need to have a drain to allow easy flushing to waste.

  • It doesn't matter whether the solids decompose in the pond or the filter - the result is the same - polluted water!
  • To maintain good water quality it is essential that solids are removed from the pond and filter before they have time to pollute the water
  • Any trapped solids must be removed from the system on a regular basis, otherwise they will simply decompose and pollute the pond. They will also encourage high levels of opportunistic bacteria.

For good filtration and water quality very little solid waste should be allowed to enter the 'biological' section of the filter. To restate the point made previously; the more effective the settlement area of the filter at removing solid waste, the lighter the load on the following biological section - provided, of course, that trapped waste is removed before it decomposes.

This entails:-

  • Regular maintenance to keep the biological area clean and free of mulm,
  • Reducing the level of dissolved organic compounds by effective settlement/entrapment, together with regular cleaning of the settlement area.
  • If we can remove solids from the system before they decompose and at the same time keep the biological section of the filter fairly clean we will;
    • Encourage a vigorous growth of nitrifying bacteria
    • Reduce the load on the biological section

Do you see a repeated theme here? Cleanliness is the order of the day!

Design Considerations

When building a filter it is suggested that either;

  • the filter surface area should be approximately one tenth that of the pond; or,
  • that there should be a pond turnover rate of once every 2 to 3 hours.

How much surface area?

Nearly all types of filtration system rely on "attached-growth" processes in which a bacterial slime layer or biofilm - comprising of bacteria, algae and often-larger invertebrates, such as worms and snails - forms on the media. Micro-organisms present in the biofilm 'feed' from the water as it flows past. So, as a first approximation, the amount of biological activity will be determined by the amount of available surface area for bacterial colonization. However, in practice this available specific surface area (SSA), as it's called, is rarely a limiting factor since most filtration systems are large.

Bacteria will thrive on almost any surface and the particular choice of medium has very little influence on their growth.

Obviously, if you had just a square piece of material measuring say 1m x 1m this would give a total area of 2 square meters (because both sides are available for bacterial colonization assuming almost zero thickness). Even this small area could support millions of micro-organisms, attached in a slimy biofilm. But typical filter media have a far greater SSA.

For instance, gravel has an available surface area of about 100 to 200 square meters per cubic meter (100-200 m2/m3). This area is also often expressed as the Surface Area to Volume Ratio, for example 200:1.

And other, more specialist media can have significantly more surface area as indicated in the table to the right.

So you can see that even a small amount of filter medium provides a potentially vast SSA for bacterial colonization.

Each square meter of biologically active surface can metabolize nearly one gram of ammonia per day, dependent on temperature, and given that most ponds will usually be producing fewer than 60g of ammonia per day, the amount of SSA required is really small - and "not a lot of people know that", as Michael Caine might say!

If we based filter sizing on the basis of SSA alone, filters could be incredibly small -perhaps only the size of a shoe box! However, there are other factors to consider....

Media SA to V Ratio
Flocor 200:1
Stone(2cm) 70:1
Kaldnes K1 800:1
Bio Block 700:1
Bio Balls 550:1
Bio bale
from CPRAquatic
Hozelock Cyprio Media 180:1
Scouring Pads 300:1
Siporax 10,000:1
Ceramic 1,000:1
Fine Foam 1,000:1
Coarse Foam 200:1
Medium Foam 600:1
Trickle Media 200:1
Bio rings (5/8in.) 340:1
Bio rings (1in.) 210:1
Bio rings (1.5in.) 130:1
Compac 3 280:1
Hair Rollers 150:1

Is void Important?

The void size or empty space within a filter medium is important in determining the right filter size and efficiency. Void size is a measure of how much of the medium consists of empty space. If we consider sand, for instance, each particle has a large surface area in relation to its volume and the total SSA per cubic meter of sand works out at thousands of square meters. Despite this enormous SSA, sand would make a poor filter medium because the small particle size would soon lead to blockages and subsequent 'tracking' as water found the 'easy routes' round the medium. And, of course, because of the dense packing, any flow through the sand would be very slow. So, despite its massive surface area, once compacted and blocked the amount of surface area exposed and the volume of water that could be treated per hour would actually be quite small.

There is another important disadvantage of a medium like sand - retention time, or the amount of time the water spends in contact with the biofilm. It is obvious that if we wish to avoid blockages and tracking, some void space in the filter medium or media is desirable. If we consider a medium such as gravel, although its larger size yields less SSA it is less prone to tracking and blocking. And specialist media such as filter matting, plastic or sintered glass have both a large SSA and a generous void space. In fact, many of them are more than 90% void or empty space! This makes tracking and blockage almost impossible.

What about cleaning?

Another important consideration - which becomes more important the longer you keep koi! - Is ease of cleaning. In the early days of the hobby, part of the novelty lies in spending weekends cleaning and vacuuming. But after a while, strangely, it seems that there are more pleasurable ways to spend a sunny Sunday. And with gravel and other granular media, it really isn't much fun trying to clean several tons of the stuff! Compared to gravel, cleaning lightweight media is a delight. Obviously, regular maintenance is somewhat easier if each filter chamber has its own bottom drain but, even so, ease of maintenance has to be a major consideration in the choice of filter medium.

The three major factors affecting our choice of filter media are:

  • Specific surface area
  • Void space
  • Cleanability

Bio-Filter Water Retention Time

Broadly speaking, the effectiveness of biological filtration is improved the longer the 'polluted' water is held in the filter - i.e. the longer the retention time. The most time-consuming process in filtration is the breakdown of dissolved organic carbon compounds into simple inorganic compounds. These compounds are ultimately incorporated back into living organisms. This complex chain of processes is not instantaneous and will, even under ideal circumstances, take some time. If insufficient filtration time is available, intermediate products will be pumped out of the filter back into the pond. This is clearly undesirable and rather defeats the object of having a filtration system. Indeed, this may well be the reason why excessive alga growth occurs in some ponds, with the filter merely producing an endless supply of plant nutrients!

So for how long should water be retained in the biological section?

This depends on how polluted the water is in the first place. Certainly, industrial water treatment plants - which handle much higher levels of pollution from sewage etc. - would retain water in the plant for many hours before it was deemed sufficiently clean to return to the nearest water-course.

Given that pond water is likely to be only mildly polluted, a retention time of 10 minutes, possibly longer, will usually suffice.

The more polluted the water is, the longer it needs to be retained in the filter. Most koi ponds will require a retention time of at least a few minutes.

How do you calculate the retention time of your filter?

Doing the Maths

This is determined by the flow rate and the volume of water in the filter. If water output from the filter is 2,000 gallons/hour and the filter contains 500 gallons (when full of media) of water then:

filter retention time = filter size / pump rate

so, in our example:

retention time = 500 (gallons) / 2000 (gallons per hour flow rate) = 0.25 hours (which is 15 minutes).

So a given sample of water will take 15 minutes to pass through the filter and back to the pond.

In the above, the filter capacity represents the amount of water in the filter - not the physical size of the filter, which will be greater.

The retention time and the size of the filter will depend to a very large extent on the type of filtration medium used.

A solid medium with low void space such as gravel will occupy much more filter space than large-pored, lightly packed media and therefore leads to a lower retention time.

More calculations!

Using our same example of a 500-gallon filter. If we now nearly fill it with gravel, the volume of water it will hold will be reduced substantially - maybe to as little as 150 to 200 gallons. Using the above example, the retention time of such a filter would now become;

200/2000 = 0.1 hours (6 minutes) or less

In comparison, if the same filter was filled instead with matting or plastic, there would be hardly any displacement and the filter will probably still hold in excess of 450 gallons, giving a retention time over double that of gravel. So a filter with a dense, low-void medium, such as gravel, will need to be substantially larger than one based on light-weight media, in order to achieve the same retention time, which explains why koi filters were traditionally so large.

Turnover - The quicker the better?

Just when everything starts to make sense, along comes a complication. While a longer filter retention time will produce better water quality we also have to consider pond turnover times. Why? Because polluted water is produced in the pond and, if there was a slow turnover at the filter, it would take longer for pond water to get processed by the filter.

To make sense of pond turnover rates it is helpful to return to the original analogy of koi being sewage-making machines: expensive food in one end and sewage out the other. Our seemingly impossible aim should be to remove this pollution as fast as it is produced. If we can manage that then we would have perfect water conditions most of the time.

When we are considering pollution the primary concern is not so much the volume of water, but rather the number of fish and the amount of food we feed to them - because this is what determines both the amount of metabolic ammonia and the quantity and quality of solid waste. There are several ways to calculate ammonia production in a koi pond. A rough and ready estimate can be made based on the amount of food fed each day.

How much Ammonia from that much Food?

Each kilogram (2.2 pounds) of fish food will result, on average, in 37 grams of ammonia being produced, together with copious feces. And there is other organic waste, such as that from decomposing algae and micro­organisms. The important point is that as the stocking, and thereby feeding level, is increased the water will have to be treated at an ever quicker rate if water quality is to be maintained.

If, for instance, we had a pond of 4,500 gallons and the fish were fed 200 grams (7 ounces) of food per day, this would produce approximately 7.5 grams (7,500mg) of ammonia per day, an average of say 300 mg per hour. (In reality the ammonia level would fluctuate throughout the day, being highest shortly after feeding).

1kg food = 37g ammonia
1000g / 200g fed = 5
37 / 5 = 7.4g = 7400mg / 24hrs = 308mg per hour.

At this feeding rate, if no ammonia was removed, at the end of a day the ammonia content of the water would be 24hrs x 300 mg ammonia = 7,200 mg in 4,500 gallons of pond water, giving an ammonia concentration of 1.6 mg/gallon or 0.37 mg/litre, which is too high.

Conversely, if it was possible to remove the ammonia at the same rate as it is produced - namely, 300 mg per hour - the steady state ammonia level would be zero. Assuming we have a perfectly sized filter, then to remove ammonia this quickly we would have to pass the entire contents of the pond through the filter every hour, giving a flow-rate of 4,500 gallons/hour, otherwise there will always be some residual ammonia present.

Deep breath!

If, instead of a flow-rate of 4,500 gallons/hour, we had a flow rate of the pond volume every two hours - or half the pond volume every hour (same thing), an oversimplified calculation would give:

300 mg ammonia / 4,500 gallons (pond volume) x 2,250 (flow rate gallons/hour) = 150 mg ammonia removed per hour, leaving 150mg in the pond, or a steady state of >0.01 mg / litre. (This makes the simplifying assumption that there is no nitrification occurring in the pond.)

What is an Adequate Flow-rate?

So what is an adequate flow­rate? As explained, it depends on the feeding rate.

The most commonly quoted advice is: turn over the volume of the pond between 8 and 12 times a day. Otherwise expressed as between one-third and one-half every hour.

Filter Size

Click here for a great resource for calculating and converting Volume.

Taking retention times and flow rates into consideration, when it comes to choosing the right filter size, there are two important but conflicting factors:

  • the right filter retention time, which ensures all the required biological activity occurs,
  • brisk water flow to prevent a high pond ammonia level.

If we decide that a flow-rate of say 2,250 gallons per hour and a filter retention time of 10 minutes are required then the volume of water in contact with the filter media at any time will need to be;

2,250 gal / 60 (minutes) x 10 (minutes retention time) = 375 gallons or 50ft3

This means that the filter should be able to hold 50 cubic feet of water after it is filled with media. This is in addition to settlement and spaces below the media trays. The required size of filter will then depend on the media used. Using a high-void medium, such as matting or plastic, we would need a little over 50 ft.3 of media to compensate for the small amount of water displacement, whereas, with a solid medium, we might need at least double the size to ensure the same volume of water in contact with the media after displacement.

Let the Water Flow

When initially designing the filter you have to apply some theory, as was just done in the previous sections. While the quoted factors provide a guide, you cannot be quite sure how accurately the pump really performs as compared to its intended statistics due to a variety of factors;

  • pump rating,
  • head of water it has to push from pond to filter, top outlet, or water feature,
  • natural flow resistance in the piping and the pre-filter and bio-filter, UVC unit, etc.
  • increased resistance over time due to solids building up if using a pressurised filter unit

After you have built your filter system, its simply a matter of timing how long it takes for the water to flow when filled up from empty.

For this experiment you should consider what constitutes the filter, and from which points to time it. For example the main priority is the bio-filter itself. Turn off the pump, then empty the bio-filter section by whatever means (drains, valves, unclip piping, etc), do them up again, and then turn on and time it.

In my case I consider the complete system to be from the inlet of my pre-filter, then into the bio-filter, and then on into the terracotta urn at the top of the waterfall because I also have some filter media in there too. So while my actual bio-filter unit is quite small, and the theoretical retention time calculated may at first sight seem too small to allow sufficient bacterial activity (the water goes through too fast), when the 'big picture' is taken into account and the effects of the pre-filter and urn are also considered, and you time it for real, then the calculated retention time can be extended by a certain factor. What do I mean by this?

Well when the water finally pours out of the urn, do you think the filtering stops there? No, of course not! As I have a waterfall, with plenty of algae formed on it, followed by a stream, then a bog area with various plants, then I can probably include these in the retention time as well because they will also have a residual beneficial action.

So I could consider the retention time to be from when I first turn the pump on, until the water starts to flow back into my pond at the end of the stream. Even then there will be natural bacterial activity on the pond walls. Another addition is that even though we have just timed the direct water flow effectively from point A to point B, there will be eddies and slower points where some of the water is held back by the filter media, plants and so forth. So the real retention time is likely to be greater than the mathematical model.

In larger filtration systems the maths is likely to be more accurate, but we know that even in small garden ponds with few fish, and without filtration that the water can be nice and clear just on the basis of balanced natural processes.

Summing Up

Ideally, what we want is a fairly brisk flow-rate, turning over the pond volume every 1 to 3 hours (depending on feeding and stocking rate) but at the same time a slow, almost imperceptible flow through the filter, allowing sufficient time for the various important biological processes to occur. Water passing through the filter should be in contact with the filter media, and therefore the biofilm, for at least ten minutes, possible longer.

There are enormous benefits to be gained by a fast turnover rate. More air/oxygen will be dissolved into the water. The solids suspended in the water will be removed more quickly resulting in a clear pond, and the fish impurities (ammonia) that are dissolved in the water will also be removed faster. Your fish will be more happy and healthy. Your water will sparkle. The colours on your fish will, generally speaking, be better.

As you increase the turnover rate the ambient level of ammonia decreases and at around a full pond turnover rate every two hours through the filter, or filters, the ammonia levels should be below what can be registered on a test kit.

As a rule of thumb a flow rate of between 60 - 80 litres (15 - 21 gallons) every minute per square meter (10 square feet) of chamber surface area will slow the water down sufficiently to give 100% conversion of ammonia in a single pass through the filter chamber but still maintain the high pond turn over rate you are trying to achieve. A slow flow rate such as this will also encourage settlement of solids.

As a working example: Pond volume about 5000 gallons. Pump needed to turn the water over in under two hours = about 44 gallons per minute flow rate (unrestricted). Filter chamber surface area to cope with a flow rate of 44 gallons per minute = about 22 - 29 sq. ft. (15 - 21 gallons per minute per 10 square ft).

You don't have to rip out your existing filter if it does not fall within these parameters - if your fish are alive and well and growing you are doing something right. You can also make some clever 'adjustments' or minor modifications to your existing filters to achieve these parameters.

So for example if you're building a 3000 gallon pond, you will need a main filter pump capable of delivering at the very least 1000 gallons per hour (3000 divided by 3). However, pump ratings are all given at zero head, the height between the pond surface and the point that water is returned to the pond. The more the head, the less the flow rate, as more power is required to push the water 'up hill'. All pump manufacturers calculate the maximum recommended head for their pumps and provide a chart giving the final flow rate at a certain height of head pressure.


Another consideration when calculating the volume of water that a bio-filter must manage is as follows:-

Add 25% to your basic volume if your pond is in full sunshine - because the water will warm up quicker, and algae will grow faster. Add another 25% if your pond is less than 75cm (2' 6") deep - because a high percentage of the water in the pond is exposed to the full power of the sun and algae will grow faster. Then add between 15-35% to your pond depending on where you live, if the pond is in the UK, it is going to be cooler and less sunny than if the pond is located in a country near the equator. If it is hotter and sunnier then algae and fish will grow faster.

This then gives us an effective pond volume, which is the amount of water we need to filter. Again the entire effective volume of the pond has to pass through it approximately once every two hours.

Lets say the pond is in full sun, in South Africa. It is 0.9m x 0.9m x 0.9m (3' x 3' x 3') so would have an actual pond volume of approx. 730 litres (160 gallons). In this example we would add 25% for the pond being in full sunshine and 35% for the lovely African climate giving a total addition of 60%. Therefore, we must filter the ponds effective volume (730 x 60%=1168) of 1168 litres (257 gallons), and any pump used should have a flow rate of at least 584 litres per hour (128 gallons per hour).

So we would filter the pond as though it had 1168 litres of water in it, and any pump we use should have a flow rate of 584 litres per hour.

If you can cut down the sunshine falling on your pond then this can greatly help reduce algae and blanket weed growth. Keeping sunshine off the pond will keep the water cooler, so slowing down their growth, and also providing healthier water for the fish. Have a look at my page on How to build a Pond Cover to protect from Herons and cut out Sunlight. It might give you an idea for something you could make that would be suitable for your pond.

My Calculations

Although my needs are very small-scale, perhaps even not worth worrying about from the point of view of ammonia production, I thought it would be an interesting lesson to perform the calculations for my setup.

First of all I need to estimate the amount of ammonia my fish produce in a day. This is likely to be quite low because I have a small pond, with only 14 small to medium fish (largest ghost koi is about 12 inches).

Feeding the Fishes

I use Tetrapond Floating Food Pellets in a 1150g tub. I wasn't sure what weight of food I was giving them each day. I only thought of it as a couple of medium size handfuls in the morning and again in the evening, possibly a little more at weekends when the grand-children like to see and feed them. So to get an accurate idea I got the tub of food, some weighing scales and found that it took 7 full handfuls (as much as I could grab) to give 100g of food. This equates roughly to 14g per handful, maybe 10g for a small fistful, more like what I actually pick up when feeding them.

So I feed them about 4 medium handfuls per day, which equates to between 40g and 60g per day. This is nothing like the figures we were using earlier on this page, which at 200g were probably more the norm for real koi-keepers. In fact when I measured out just 100g of the pellets I was surprised at what a large bowlful it was. As an aside if I feed an average of 50g each day, the tub at 1150g therefore should last me 23 days. In fact the tub lasts much longer than this (more like 2 months), and I think its because I am more of a sucker when feeding the fish, than when my wife feeds them!

Now lets calculate the daily ammonia:-
1kg food results in about 37g ammonia excreted by the fish
1000g / 60g maximum fed each day = 16.6
37g / 16.6 = 2.228g = 2228mg / 24hrs = approx. 93mg per hour.

As stated earlier each square meter of biologically active surface can metabolize nearly one gram of ammonia per day. My fish produce > 2g each day (2228mg). Lets say 3g to be safe because they probably forage and eat other food in the pond, although it will be of very low protein content. So I need an SSA of at least 3 square metres. I'm using scouring pads as the media with a ratio of 300:1, or 300m2 for each cube metre of media. Imagine a box 1m x 1m x 1m, its quite big. But now imagine a big flat square 300m long and wide (think 3 football pitches long on each side), now mentally fold up that area and pack it into that box. Thats quite a remarkable area to fit into the box!

So 1 cubed metre / 300 = 0.0033m3 (or 3300cm3). That is 3300cm3 required per m2 per gram of ammonia. We reckoned on 3g ammonia, 3 x 0.0033m3 = 0.01m3 (10,000cm3) of filter media. The square root of 10,000cm = 21.544cm, so roughly a 22cm length x 22cm width x 22cm height cube would be a sufficient amount of media to handle the ammonia. Scouring pads don't come as one big lump though, so they would actually need a slightly larger container!


My pond contains about 400 gallons, and my pump is rated at 616gallons/hour at 1m head, although its maximum flow rate is quoted as 4000litres/hour (880gallons/hour). I will add 25% for the pond being in full sunlight, and 15% because I am in the UK, so giving 40% and resulting in 560 gallons effective volume. Therefore I need a flow rate of only 280 gallons per hour (half the pond volume), or about 4.6 gallons per minute for recommended pond volume turnover.

My waterfall is at about 1m above pond surface level, so in fact my pump at 616gph / 560 = 1.1. So every hour the whole pond water would be recycled one and a bit times, or about 21 times in a day. Thats a very fast turnover considering the recommended is 8-12 times a day. In fact it will be less than that because my Cascade pump also has a bell-effect water fountain, so some of the water just cycles in the immediate vicinity of the pump within the pond rather than going to the filters. Perhaps I can use my fountain to help reduce the power and flow of water that is directed to the filters so as to increase the retention time in the filter, but I have a feeling that my stream would then be little more than a trickle. Catch 22!

Retention Time

Hmmm. I have a feeling this is going to be ridiculously small. Suppose the bio-filter were based on a 22cm cube (the figure calculated for SSA earlier for the ammonia was 10,000cm3). This would hold about 2 gallons of water. But the pump at 616gph would push the water through in about 12 seconds!! Thats a far cry from recommended RT of 10 minutes. Even if I reduced the pump flow to my desired 280gph, that still only gives a 25 second RT.

So lets work it out the other way, to figure what size the bio-filter should be to obtain 10 mins RT:-
280 gal flow-rate / 60 (minutes) x 10 (minutes retention time) = 46 gallons or 7ft3 (about a 59x59x59cm cube).

What does this mean? We seem to have a contradiction in that the amount of ammonia my fish produce suggests I need a filter capable of holding only 2 gallons, and yet to hold the water for 10 minutes, I need to increase its size to 46 gallons!

When I looked at the size of commercially produced bio-filter units rated for my size of pond I thought that as long as I build something of similar size I can't go too far wrong. What surprises me however is that the theoretical calculations as described on this page, when applied to the dimensions of such commercial filter units, indicate that the small-scale units are far too small to achieve anything near a 10 minute retention time! Also when you consider that these units usually have half their volume as pre-filter, and the other half as bio-filter, and often using a plastic filter media with a lower SSA ratio of say 180:1, then I am skeptical of them achieving any benefical biological filtration because the water passes through far too quicky.

Surely they can't be too far wrong though? I can surmise in my situation that ultimately the filters main function is going to be creating clear water, rather than reducing ammonia.

The Real Thing

When it comes to the actual practicalities, and the size of container (big flower pot) I have chosen it actually works out more like this:

The pot is a cylinder of average diameter 38cm (it narrows slightly from top to bottom, so I take the measurement at the middle). It has a filter media depth of about 32cm (the pot is actually taller/deeper but the media itself is 32cm deep). To calculate the volume we use Pi x radius x radius x depth, or 3.1415927 x 19 x 19 x 32 = 36291cm3 = 8gallons. A flow rate of 616gph would have a retention time of 46 seconds, and if reduced to 280gph would be 1 min and 42 seconds.

As mentioned earlier, after building the filter system, time how long it takes for the water to flow when filled up from empty. I did a test and timed my completed system: pre-filter, bio-filter, water-feature urn, waterfall and stream.

It takes 4 and a half minutes before the water spills back into the pond again, which is nearer the goal of 10 minutes retention time, and I believe is probably adequate for a pond of my size.

Next stage is to build it!

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