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Jim's Crazy Bio-Filter



Hi - Welcome to my DIY Fish Pond Bio Filter Pages! :o)

I hope you find them useful.

What you get: In this project I give some background into my pond, and the common "green pea soup" algae problem (see Essential Reading below), my personal opinion of pumps and filters on the market and some modifications. I cover construction of a DIY BIO-filter with a simple venturi system and vortex chamber, and identify key issues in preventing algae, and also provide some useful resources and forums for research.

Disclaimer: I am not a professional, and my small pond is not specifically for Koi fish, this project is purely suggestions from my own experience and research. Some aspects involve modifications to equipment that could invalidate your warranty. I just enjoy trying things out, and judging by the number of people looking for similar information in fish pond forums, as one of them you might be interested in my ideas.

Safety: Please be careful! If you have a young family, do remember that kids are fascinated by water. You really should think twice about building a pond if you are unable to supervise youngsters at all times. Tiny tots are fearless and don't understand the dangers of water, and it only takes a minute for a little person to drown, even in very shallow water. Having said that, our grand-children visit regularly, and with constant reminders they learn to be careful. But we never let the youngest ones near the pond without an adult right by their side.

Our pond, complete with lighting, and fake heron!

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.

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

First things first

Before you get stuck into reading about my bio-filter project you are probably asking yourself "Is it worth the effort?". In the last couple of years since adding the bio-filter my pond water has remained crystal clear ALL year long, even through the summer.

You have probably read some stuff elsewhere about other people trying their hand at building a bio-filter (a home for good bacteria to live and work), and want some kind of reassurance. If you have a pond without a bio-filter and you are experiencing green water or fish health problems then you really should install one.

I would also like to make it clear to you the importance of having either a venturi or an airstone to oxygenate the bio-filter. Oxygen is the magic, the wow, the X-Factor in a pond. You need oxygen to survive - your fish need it too! And your bio-filter will be less efficient without a good supply of oxygen. Also if green water is currently a big problem for you then at night the algae will deplete the oxygen in the water and your fish may suffocate! Green algae in itself is not bad, fish like to eat algae of various sorts, but when a bloom becomes excessive it will compete with your fish for the oxygen. Photosynthesis in plants (and algae) absorbs carbon dioxide and gives out oxygen during daylight hours, but the process reverses and uses up oxygen whilst producing carbon-dioxide at night. This can cause distress for your fish.

There is a lot that goes on in a ponds bio-cycle and your understanding of this will greatly help you achieve miracles with your pond. I will tell you about all this in due course, but I thought I would just reproduce the following section here now, which comes from right at the end of this project. This is my own proof and experience, but I have also had several other visitors to my web site email and tell me how pleased they are that once again they have clear, clean water in their pond, using the simple principles explained here, and without the need for expensive manufactured units.

The following taken from my Was it Worth It section....

"During the first 2 weeks of experimenting the bio-filter didn't really get much chance to settle down, what with me pulling it apart every couple of days to re-arrange the venturi. It is said that a bio-filter requires about 6 to 8 weeks to mature, while the bacteria builds up, and a balance is created in the ponds eco-system.

In the third week, with the early prototype venturis not working very well, there was very little happening, and while the water was beginning to look cleaner due to solids filtration, it was still VERY green.

But in the fourth week, about 4 days after I got the final venturi design working mixing air into the water nicely, the water was starting to clear noticeably. Up until now the fish could only be seen clearly if they came within about 3 inches of the surface, any deeper and they became a green blur. But after 6 days of oxygenating the water going into the bio-filter the fish could be seen progressively deeper, now down to 6 to 8 inches, and the vivid green was weakening.

Now about 8 weeks after the whole bio-filter and venturi setup has been fully operational we can once again see our fish. The water is much clearer and cleaner. The bio-filter has healthy looking gunk in it, and the pre-filter does not seem to be blocking up as quickly - I still clean it regularly, but there is just not as much build-up of muck in it."

So, are you sitting comfortably? Then let's begin .......

When we moved to a new house in 2001 we built a fish pond. My wife didn't want anything too big, while I on the other hand, had bigger ideas! Our garden pond was to be a home for a few goldfish (not a large koi-pond project), although as time has passed and the fish have grown, it has started taking on some aspects of the koi-keeper's concerns.

When we were planning the layout we started by using bricks on the lawn to mark out the shape and size. For about a month we had fun battling out against each other the actual size of the pond, which grew (me), then shrunk (wife), every other day, until we agreed to differ.

This is the fruit of our labours after 3 years. At first we just thought it would be mildly interesting, but in this time we have grown to love our fish, and we get many hours enjoyment maintaining and improving it, even more so as our little fish have grown into quite size-able whoppers!

We would even venture to say that from daily observation, and getting to know them, that our fish actually have different "characters". Maybe, as keen Scuba Divers who have seen many wonderful and beautiful forms of sea life, we have formed a respect and affinity with our aquarian friends in their own little garden haven.


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!

In a Nutshell

The original plan was always to have a "terracotta water urn" spouting water into the top of a small stepped waterfall, leading into a little stream and "bog" area, then into the main pond.

We didn't want a prefabricated waterfall. Many of the fibre-glass or plastic waterfalls available were either not interesting enough, the wrong shape for the layout we wanted, or looked un-natural.

Most items used for our pond were bits and bobs that we found in various places, and the millstone style edging was a £50 batch lot picked up from the junk section of a local stone merchants.

We used high-grade pond liner to line the pond dug into clay (hard work by hand!), edged with substantial wooden battens to prevent cave-ins, and hold the weight of the millstone edging, and levelled using the old "water in a hose-pipe with two funnels" trick.

View of the "urn" at the top of the rockery, spouting into the waterfall, down the stream, through the bog-area, and into the pond.

There are 3 levels:-

  • Deepest at 3ft. has a Hozelock Cyprio 240v 4000litres/hour fountain pump sat on a couple of bricks (now replaced by a Titan 8000litres/hour on the bottom), which powers a "water-globe" nozzle (bell) fountain (which doesn't foul up as quickly/easily as a fine water-jet fountain), and also pumps water up to the top of the waterfall (originally via a Hozelock Bio-Force UVC Filter although this is no longer connected) and my home-made "Skippy" style BIO-filter (the construction of which is described in this project).
  • Middle at 2ft to give some depth below the streams entry into the pond, so the fish can swim happily underneath, and the shelf can hold mid-depth aquatic plants, plus the shelf can be used to move the pump to a higher level in winter to allow warmer water to stay at the bottom.
  • Shallow shelf at about 9-12 inches around the pond-perimeter for marginal aquatic plants (although we have discovered to our distress that herons love this sort of depth to stand and fish in!). We now have a slatted wooden pond cover over the pond.

Building a Small Stream leading into the Pond

One of things that really makes a big difference to the enjoyment of our pond is the little stream and bog area. I have added some more information about this, and so have created a new page on its own. Click here to learn how to build a stream leading into your pond.

It's well worth the effort. A stream and bog area attracts so much wildlife; we get frogs, dragonflies, water-boatmen. Also its a delight to see wagtails and blackbirds come to wash in the stream. On occasions when heavy rain has raised the water level in the pond to overflowing, the smaller fish will swim up the stream to explore.

Green Pea-soup

During the second year our fish had babies, and the adults were growing at an amazing pace. We started with 12 fish, a mix of golden orfe, sarasa comets, shebunkins, and ghost-koi, all of which were about 3 inches or smaller. Now the ghost-koi look fantastic and the biggest is about 12 inches long and looks very stocky and chunky. I'm sure he'd make a good dinner (only joking).

For the first year the water was lovely and clear, even in the summer, and we had good cover to provide shade from the marginals, like lilies, water hyacinth, and "Fairy Moss" - that red-leafed surface cover weed that can take over the pond and you end up scooping out handfuls of the stuff. Also in the boggy area of our stream we had a good variety of bog-plants; Marsh Marigold, Horsetail Rush, Water Musk.

The main pond, now covered with netting since we lost 2 beautiful carp to a heron.

But, in the second year (Summer 2004) things suddenly started going wrong.

Three things seem to be the culprit:-

  1. The fish were much bigger, and eating more, were therefore excreting more.
  2. Even though we put more of the Fairy Moss in the pond to get more cover to keep out the sun, it didn't survive, we think the fish were eating it(?).
  3. The yellow-flowered Water Musk had pretty much taken over the entire bog-area, and so we had a mad fit and removed ALL of it!

Ooops! Within about 2 weeks the water went a pea-green, taken over by fine water-born algae. We couldn't see below about 4 inches, any deeper and the fish could only be made out as dark shapes passing below.

In hindsight we now realise that the plants in the bog-area, with their massive root network, were acting as a wonderful natural "veggy-filter" to eat up nitrates in the water, hold back any crap being pumped out of the pond coming back down the stream, and provide a home for bacteria that was helping the whole "balance" process.

When we designed our pond, waterfall, stream and bog-area, we didn't really appreciate what a clever design we had given "nature" to work with.

And we had just messed up by removing the most vital part of the cycle. A natural vegetable biological filter.

Initially we didn't figure out why it had happened. At about the same time the fish had been traumatised by the heron eating two of the larger fish, and we thought the water was cloudy because they were stirring up the bottom silt to "hide themselves".

I was also starting to get very fed up with cleaning my Bio-Force filter literally every 2 to 3 days, and having to remove the Cascade water pump from the pond every week because that too was getting clogged up, mostly by blanket weed.

I knew that sunlight was going to be a major cause of the algal-bloom, but I was starting to get concerned about the state of the water and the health of the fish, the water at times was almost looking "brown/black", and so started my quest to learn more about what was going wrong and how to resolve the problem.


My web site has a lot of useful information, but the one thing I would most want you to gain from visiting this site, is simply to understand life, the nitrification cycle and why you need a bio-filter. For this you should read the following section:-

Introducing "Skippy"

Everywhere I looked there seemed to be plenty of "remedies" in the form of expensive solutions and additives which promised to eradicate the blanket weed algae that was fast overcoming the pond, and other remedies to remove the water-born algal bloom. However I was already trying these to no avail.

It seemed like a threshold had been passed where the algae was winning hands-down.

I had been toying with the idea of buying a new bigger better pump, and getting a better filter (although at this point had no idea what kind), but all this seemed yet more expense given that I had already upgraded from a Hozelock Cyprio 24v Low-Voltage 2000litre/hour pump to the 240v 4000litre/hour model earlier in the year, and also purchased the Bio-Force UVC (Ultra-Violet Clarifier) 2200 Filter, which now I didn't think was man enough for the job (even though the performance ratings suggested it should be sufficient).

I also felt that the Ultra-Violet clarifier was just a gimmick because it didn't appear to be having any effect on removing the algal bloom, even though my pond at about 400 gallons is by no means large as compared to some peoples. [In fact 2 months after finishing my bio-filter I had clear water and the UVC bulb is no longer connected!]

So before making any impulse purchases I felt it would be wise to do some research into the best solution. I started doing some surfing around all kinds of pond-related web sites, most of them obviously trying to sell their stuff, and many of which didn't appear to give the answers I was looking for.

Now before going any further into my project I would like to show you a web site which I thought was a breath of fresh air amongst all the technological gobbledy-gook being pushed at me by the myriad of web sites whose sole purpose appears to be providing "good advice" with the ultimate aim being that you buy one of the multitude of different makes of pumps, filters and other accessories.

Skippy's is the site that gave me the inspiration to have a go at building my own BIO-filter. And the most wonderful thing is it explains all about "freakin magic"!! You NEED to know about Freakin Magic in order to understand ponds (you'll see what I mean if you don't already know), in fact you'll learn about Nature and LIFE! In fact even for experienced pond keepers, it makes a great read - the emphasis is on helping you understand how to work with Nature to create balance, not against her.

Skippy's then tell you how to build your own DIY vortex/settlement-chamber based BIO-filter. Its a great site but for one thing. I don't think they make it clear enough that you need to PRE-filter the water going into the BIO-filter (maybe I missed something!). For a novice like me I got the impression their bio-filter was able to do everything. It wasn't until after I had built my basic bio-filter based upon their ideas, and then later read in their associated forum about other newbies who similarly misunderstood the need for PRE-filtering, that I made some changes. (I also emailed them about it and they confirmed I was right).

Well, this project is my own take on the Skippy Bio-Filter. Once you've read their web site (and I strongly recommend you do), if you are interested come back and see how I've done it (and implemented a simple venturi too - a mechanism to mix air into the water with no moving parts), and also where to get filter media for your Skippy filter if you're in the UK (Skippys is a USA site).

So here it is ...... Skippy's bio-filter - take your time reading and absorbing these 3 essential pages (I find them fascinating and educational, and have often gone back to them to remind myself what we are trying to achieve when creating a bio-filter).

I suggest you start first by reading this page, closely followed by this page to learn about Freakin Magic, finally and probably most important is this page about TIME, then you can go to this page for their Bio-Filter construction.

Remember to come back here! :-)

Now I will show you what I have actually done, and why. Click here for the next stage of the project.

If you have any comments or suggestions about this project please contact me:

And if you have found these web pages useful and interesting and would like to make a donation, you may do so using the Donations Page.

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

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Here is a short glossary of terms:

A biological filter is a vital part of any pond system. This chamber (or chambers) follow the mechanical PRE-filter. A good, long-established biological filter should be home to a colony of "aerobic" bacteria that convert toxic ammonia to less harmful nitrates. This bacteria can be boosted with additions of fresh bacteria culture, say at the start of the season, or after a new bio-filter has been installed, and can be obtained from any good fish pond stockist.

A venturi is a device that injects air into the pond water. It is made very simply from pipework and has a restrictor inside and an air tube which extends above water level (you will see a diagram later on). Water is pumped through the venturi, and the restrictor forces air to be sucked in from the tube above water level, and then mixes with water inside the venturi to cause thousands of tiny bubbles that aerate the pond water. It is used to improve aeration and movement of water in the pond, and increase the oxygen level in the water.

A settlement chamber is the lower part of the filter, to which waste is channeled. The solid matter sinks to the bottom.

A vegetable filter containing plants like arrowheads, water lettuce and water hyacinth, will absorb harmful nitrates and phosphates from pond water.

Vortex: when this cone-shaped unit is installed, the pipework from the bottom drain is connected to it. When the valve is pulled, the water enters the chamber in a whirlpool action drawing heavy solids down in a gravity-fed, clockwise flow to the steeply-coned base of the vortex. By pulling the vortex valve, these solids are flushed to waste. The water exits the upper part of the chamber via a pipe towards the next stage of filtration.

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