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How to Construct a Homemade DIY Pre-Filter

This page shows you how I made my own design of pre-filter to use with my "Skippy"-style bio-filter. Below you can see two photos showing the bio-filter before and after fitting my pre-filter.


After - Yes, that white pipe contraption!

Ok, it looks a little odd, but it works well for me. The pond pump pushes water from the pond via a control valve (which lets me adjust the pressure going into the filters), into the bottom of the pre-filter, up through some filter media, then out via a T-junction side-ways into the bio-filter. I have had it in use for several months, and it definitely helps keep larger muck from entering the bio-filter. However it may not be suitable for everyone - I will say why in a moment.

First I will explain why I created this design.

  1. First some history - In some of the other pages on this website I explain how I got fed up with my Hozelock Bio-Force UVC filter because it was too small for my pond (green water was a problem), the coarse/fine foam filters would block up frequently and it was a right back-breaking, fiddly, pain in the butt to clean regularly. So I decided to build my own design of Skippy filter as the main filter but for a while continued using the Bio-Force to pre-filter the water before the bio-filter - until I got totally fed up with cleaning it so stopped altogether. So for a while my Skippy bio-filter had NO pre-filter to keep muck out of it!
  2. After a little education from the original Skippy Bio-Filter web site I changed my attitude on pond maintenance from trying to do as little and infrequently as possible, to making sure I was cleaning things regularly - because that means happy healthy fish. But to encourage yourself to perform regular cleaning means you want to make things as quick and easy to clean as possible.
  3. The Skippy web site told me that having a pre-filter before my bio-filter would help its long-term performance, because there is less muck going into it, which means less fish solids decaying in the bio-filter (solids require anaerobic bacteria to dissolve/eat it up - not the kind we want), and so by filtering out the solid muck, it makes life easier for the aerobic bacteria to develop which are the good kind of bacteria that turn poisonous ammonia (excreted by fish), into nitrites, and then convert nitrites into harmless nitrates which plants can eat up.
  4. Have you ever been on a boat holiday? A motor cruiser on the Norfolk Broads or the Thames? If so you might remember one of the daily maintenance tasks is to clean the water filter for the boats engine, which pumps cold water from the river to keep the engine cool. The filter prevents river weeds from being drawn into the engine cooling system, but each morning before you start the engine you have to unscrew the top of the filter, remove the metal gauze and clean it, then pop the gauze back in and screw the top back on. Well this is the kind of thing I had in mind for my design, except of course using PVC piping and fittings rather than metal which might contaminate the water.


  • Solves the problem of not having an out-of-pond pre-filter before a Skippy style bio-filter. DIY'ers who build their own bio-filter often tend not to have any kind of pre-filtration, and this ends up with the Skippy becoming gunked up and less efficient over time. Some people rely on a pre-filter cage over their pond-pump, but this can require regular removal of the pump from the pond for cleaning. I prefer to have a low-maintenance solids-handling pump and deal with the first stage of filtration out of the pond in my easy-clean pre-filter.
  • Fairly easy to construct using basic tools
  • Because it has a only small surface area in which to trap weed and sediment, it can clog up fairly quickly but this doesn't matter because cleaning it is dead easy! Even so it doesn't clog up quite so easily as foam does.
  • Uses the same type of green scrubbies (nylon pot scouring pads) for filtration as I bought to use for the bio-filter media
  • Is at a nice height to maintain - no more bending over fiddling with Bio-Force clips, big o-rings, foam and bending flexible pipe until eventually it breaks (you'll know what I mean if you've read my other pages).
  • Is quick to unscrew the top, remove the filter, rinse with water, re-insert and screw the top back on again. This is a 2 minute job, and 1 minute of that is a leisurely stroll from the pond into my utility room to clean the filter mechanism under a running tap. So I really don't mind if I have to clean it daily in the summer, although that is my own preference - it will last for a couple of days without cleaning. In winter it only needs cleaning once every 4 to 6 days.
  • Works with my existing design of bio-filter
  • Bacteria solution can be poured into the top of the pre-filter and "injected" right into the heart of the bio-filter to boost bacteria levels after cleaning or topping up the pond.


The pump I use nowadays is fairly powerful. I have a Hozelock Titan 8000 litres/hour solids-handling pump. This is a beast of a pump which I throttle down using a control valve. In fact I have two valves near the top of my waterfall.

The 25mm flexible hose from the pump leads into a T-piece (top-left).

The T then leads one direction into a valve (bottom of photo, its closed in this shot) which controls the rate of flow into the pre-filter, then bio-filter, then exits to the top of the waterfall.

The other side of the T leads into a second valve (top of photo, about half open in this shot) which acts as a bypass straight to the waterfall.

This means I can use the valves to balance the flow either to the filters or the bypass. I adjust the flow so its correct into the bio-filter (fast enough to operate the venturi to mix air into the water as it enters the bio-filter, but slow enough for the bacteria to do their work), while excess flow can go to the waterfall to give the stream a nice convincing flow.

I tell you - with this pump and the valves I have all the power I need to operate things efficiently for my pond. I really really wish I had bought this pump in the first place. But I learnt the hard way and went through 2 lesser pumps initially (but they are not wasted because I use them for other purposes now). If I turn both valves fully open the flow down the waterfall and stream is enough to wash away gravel and small pebbles!!!

Anyway - back to the point in question - the possible disadvantages.....

My pre-filter design might not work so well with a pump less powerful than say 2000 litres/hour because as the filter catches the muck it will block and restrict the flow of water. Something less powerful may not be able to overcome the uphill height, and push through the pre-filter. Actually I have found that the green scouring pads in the pre-filter are great for trapping large particles of weed, fish crap and uneaten food, but actually do not restrict the flow of water too much. Well, certainly not as quickly as foam/sponge does. Smaller particles pass through and are caught by the vortex settlement chamber of my bio-filter design.

Remember that with this design I am happy to clean the pre-filter on average once every day or two, and far less frequently in the winter - but its so easy its hardly a disadvantage! The only thing that might make it awkward to clean frequently is if the layout of your pond and garden makes it difficult to get at the pre-filter. But only you can be the judge of that. And the layout of your pond equipment for easy maintenance should be a strong consideration right from the start anyway.

If someone tries building my pre-filter and uses it successfully with a smaller pump I would love to hear from you. I'm not saying it won't work - just bear it in mind before you go buying the parts to make this pre-filter! I think I spent about £23 on the various parts.

So if this sounds useful to you, read on to learn how I constructed it....... then adapt the design to your own needs.

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.


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!

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

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Building the Pre-Filter

Here you can see a side view of the pre-filter assembly which shows clearly the use of a self-cutting outlet kit (the grey bit) which directs the water into the bio-filter.

The topmost section is a 40mm pipe Universal connector which provides the screw-top and access-plug which you undo to remove the filter-pads for cleaning.

Right at the bottom (not visible, see further down this page) is a 90 degree bend which the flexible hose from the pump connects onto.

The water passes upwards from the bottom, through the vertical pipe body of the pre-filter, passing through the filter-pads which are held in place internally by a metal rod and plastic spacers, and then out through the outlet kit, which feeds via a 22mm diameter straight overflow tank connector into the side of the bio-filter.

Side view of my pre-filter

The main parts used in the construction.

  • 1 x Self Cutting Outlet Kit - adaptable to 32mm or 40mm diameter piping
  • Length of 40mm diameter plastic piping 1 metre should be sufficient, but its usually sold in 2.1m lengths, the actual length depends on how much clearance your pre-filter has from the top of the bio-filter down to ground level. My bio-filter sits upon a couple of house bricks, which are on top of a paving-slab platform, providing a good solid base which raises it above ground level by a foot or so.
  • 1 x 40mm diameter Universal Connector (comes with set of rubber and plastic sealing rings)
  • 1 x 40mm to 32mm diameter Reducer (comes with set of rubber and plastic sealing rings)
  • 1 x 22mm diameter Overflow Straight Tank Connector
  • 1 x 22mm diameter Overflow Bent Tank Connector
  • 1 x 40mm diameter Push-fit Access Plug
  • Set of 40mm diameter Waste Pipe Clips (shown in photo, but were not actually used)

The most expensive part was the Self-Cutting Outlet Kit at about £8. You can see that I got all these parts from the local Focus DIY store (in the UK). I expect other countries will be able to supply same or similar types of fittings.

Before we go any further, its time to familiarise you with the internal workings of the pre-filter, so that you'll understand how it all fits together, and where each part is used. So here is an exploded view of all the parts. Don't worry, it only looks complicated because of all the extra washers and o-rings.

Now each piece in turn:-

Self Cutting Outlet Kit

This is the most complex and expensive part. Normally this is used to allow easy plumbing in of the waste water outlet flexible hose from a washing machine into a sinks drain pipe. It also prevents back-flow of dirty water from the sink into the washing machine by means of a non-return valve.

The clamp holds onto the 40mm piping by means of 4 screws and a rubber sealing pad. Once its position has been measured and it is clamped into place, you use a special cutting tool (the black bit at the top of the bag, this is supplied complete with the kit). This is used to cut neatly through the wall of the pipe. Don't cut until you know exactly where you want it positioned on the pipe.

Also included in the kit are a variety of connectors and bends, and a special non-return valve.

Not all of these parts were required for the pre-filter assembly and so a couple were just put in my spare parts box for some future purpose!

Universal Connector

The Universal Connector fits onto the top of the main pipe, and uses the rubber seals and plastic o-rings to make a good watertight seal.

The bottom screw section is screwed very tightly onto the main pipe so it never comes undone, while the top section is used to screw on the Access Plug. This is done up only moderately tightly because it is this screw top and plug that you will undo and remove every time you need to clean the filter.

Access Plug

The Access Plug is simply a blank-plug used to block off 40mm piping. You should find one of the O-rings supplied with the Universal Connector will fit over the Access Plug.

As mentioned above it is the bit you remove each time you need to clean the pre-filter.

Below it is shown the.........

OverFlow Straight Tank Connector

Originally my bio-filter had a different connector in the side (see pic below), but that particular fitting would not have connected to the Self Cutting Outlet Kit.

Old connector - no good

So I replaced it with this new 22mm Straight Tank Connector (pic at above right)

Note: When installed in the bio-filter the left hand side of the connector as shown in this photo was facing outward, and the right hand side was inside the bio-filter.

The 22mm pipework inside the bio-filter (which leads into my homemade venturi unit), simply push fits into this connector - the screw-up cap was not necessary.

These are just example pics showing the pipe push-fitted into the connector (left), and my venturi (right).



Straight Tank Connector - LH Side outside, RH Side inside bio-filter tank




In the top-half of this photo you can see how the 40mm Universal Connector, Access-Plug and sealing rings fit together at the top of the pre-filter pipe. (Ignore the bottom half of this photo for the moment, I'll explain it later).


Universal Reducer

The Universal Reducer is normally used to change from a 40mm section of pipe down to a 32mm section of pipe.

This Reducer attaches onto the bottom of the main pre-filter 40mm pipe.

One of the difficulties I have found when shopping for parts is making pond pipework marry up with standard domestic plumbing fittings, and in this instance I needed to go from 40mm pipe right down to something that the 25mm flexible hose from the pump could connect to.

So I fashioned it out of this Reducer fitting. I found that the 32mm end of the reducer had a screw-cap with an opening that was suitable for the 22mm Bent Tank Connector and rubber washer to screw tightly into, and then the screw-cap (complete with attached 22mm Bent Tank Connector) simply screwed onto the Reducer.

Despite the unusual method, this has proved to be a rigid and watertight connection.

Look at the photo below of the Bent Tank Connector, and imagine the screw-nut with the black rubber ring going up into the bottom of the Reducer pictured here to the right. The screw-nut and rubber ring will go on the INSIDE of the Reducer screw-cap. Long-nose pliers are needed to do them up tightly because your fingers can't get in between far enough.

Here is a closer exploded view of the parts for the bottom section of the pre-filter. It shows the order and orientation that the parts need to be put together.

40mm Screw-connector, Plastic washer and o-ring------------------------>

40mm to 32mm Reducer--------------------->

32mm Reducer O-ring -------------------------------->

22mm Bent Tank Conn. Nut ------------>

22mm Bent Tank Conn. O-Ring ----------->

32mm Reducer Screw-cap with hole into which the Bent Tank Connector is fixed ------------------>

22mm Bent Tank Connector --------------->

Complete Bottom Section

Here you can see the finished bottom section of the pre-filter, comprised of the 40 to 32mm reducer and the 22mm Bent Tank Connector.

It is best to assemble the bottom section in readiness, but don't connect the flexible hose onto the Bent Tank Connector until very last after you have fitted the pre-filter to the bio-filter. This way you know the exact position and length of flexible hose you will require.

The flexible hose coming from the pump and control valve is 25mm, so its not quite a perfect fit onto this 22mm fitting which also has a thread, so a Jubilee clip and plenty of Fernox-LX plumbers jointing compound was required to make it good and watertight.

It was a bit fiddly to make the flexible hose connection good, but once done up good and tight this has remained waterproof.


Self Cutting Outlet Kit Assembly

Here is a closer view of the outlet kit. So working from left to right, note the following points:-

You must leave enough distance (about 40-50mm) from the top end of the main pipe down to the top of the Outlet Kit Clamp. This is so there is enough pipe to go fully into the Universal Connector and allow its screw-connector to be done up. Make sure you get this distance correct before clamping using the 4 screws and cutting through the pipe. The black "bit" with the chrome bar through it is the cutting bit. You slowly but firmly screw it by hand into the threaded hole in the clamp, and its curved cutter slowly bites its way through the 40mm pipe.

[Note: The white straight tank connector at right of this photo is shown the wrong way round].

The next 3 grey parts in the middle of the photo are;

1) The non-return valve body (left),
2) The non-return valves shut-off o-ring holder (middle with black o-ring) - this part is also threaded internally and accepts the thread from the white straight tank connector shown in the above photo.

3) The non-return valve screw-head.

Now look at the photo below. This is a close-up of the non-return valve. You can see how it is comprised of the body (top), and inside there is a white plastic cap and a small steel spring in a mount (middle of picture). The cap normally seals against the black O-ring in the O-ring holder (bottom), which simply push fits into the non-return valve body, and is secured by screwing on the non-return valve screw-head (not shown in this photo).

Right then - that sprung non-return valve mechanism would normally be inside the non-return valve body. But using some long-nosed pliers I deliberately broke it out!! We don't need it, and it would prevent the flow of water because normally water flows through the valve in the opposite direction to how we want it to work, so get rid of it.

Screw only the non-return valve body into the main pipe clamp - nice and tight. Leave the non-return valve o-ring holder and non-return valve screw-head off for the time being, these will be used when ready to attach the whole pre-filter to the bio-filter.

Attaching the Pre-Filter to the Bio-Filter

Screw the white tank connector into the side of the bio-filter body, using appropriate washers to help achieve a good seal, and do this up nice and tight. A dab of Fernox LX sealant may help prevent any leaks.

Push the non-return valve screw-head over the white tank connector making sure its thread faces the right way towards the pre-filter assembly, then screw the internal thread of the non-return valve o-ring holder onto the white tank connector thread. Do this up as tight as you can.

Now apply some vaseline to the outer face of the non-return valve o-ring holder (this will help achieve a waterproof fitting), then offer up the whole pre-filter assembly which will just slide onto the o-ring holder. Holding it in place simply screw the non-return valve screw head over the o-ring-holder onto the non-return valve body, and do this up tightly to finish.

After doing everything up tightly I found that the whole construction supports the weight of the pre-filter without requiring any clips. This is just as well because it is too far away from the body of the bio-filter to clip it on anyway!


Filter Media Assembly

Now for the actual filter media assembly, which you can see in the picture below. This is constructed using:-

  • 10-12mm diameter black plastic tubing - about 20 inches long.
  • Plastic coated very stiff metal wire or rod (coat-hanger wire)
  • Thick green scouring pads as filter media (must be suitably thick, about 1/4 inch or 8mm)

This is constructed very simply out of a length of very stiff plastic coated wire or rod. I used coat-hanger wire, because it is very rigid, yet can be bent with a pair of pliers. At the very bottom, the wire is bent to form a flat ring section which holds the filter media in place and prevents it falling off the bottom. Similarly you can see the wire is bent over at the top to hold the whole thing together (but the hook at the top still allows the black tubing, spacers and filter discs to be removed for adjustment or replacement). The hook is also handy for your finger to grab to pull the filter assembly out.

The filter media is comprised of 18 discs cut out of the green scouring pads. These pads MUST be quite thick. After some time the nylon material softens and bends over at the edges. If the pads used are too thin, they will be ineffective because they will become too flimsy, and the force of water would simply push straight past them.

The black plastic tubing acts as a handle at the top, about 5 inches long. The length of this spacer section should be made long enough to ensure the filter media is below the water outlet clamp when the filter is fully pushed home and the lid has been screwed on.

Then the tubing is cut into 17 short sections which act as spacers between the 18 filter media discs. Each short section is about 8-10mm long. It doesn't matter that these spacers wobble about on the central wire, the water will pass through them and they simply prevent the filter media from bunching up. If they bunched up then pond weed and sediment would block it faster.

Important Note: As mentioned above, these discs soften up after a few days and start to bend over at the edges so you need to cut them with a diameter about 10mm more than the internal diameter of the pipe, ie. an overlap of 5mm all the way around. This is so they have a nice tight fit inside the main pipe. When you push the filter assembly into the filter pipe for the first time, it will be fairly tight and quite hard to push in. Don't worry - this will become much easier when it becomes wet, and after 2 to 4 days of use the nylon will soften.

In fact I later made a small adjustment to the filter disc pads by cutting the ones at the bottom to be a slightly narrower diameter, so as to give a slight taper from the bottom to the top. This helps to prevent the filter from blocking too quickly on the first few discs by allowing some muck to squeeze past and be trapped further up the filter.

Maintenance Cycle

How regularly you need to clean the pre-filter depends on a few factors;

  • Time of Year - summer requires more regular cleaning, winter less cleaning
  • Number of Fish - more fish means more sediment
  • Amount of water-borne weed - obviously several factors can affect this, e.g. if you stir up your pond as part of cleaning it, if your fish stir up the pond, amount of blanket-weed, etc.
  • Power of your pump - slower flow seems to allow the filter to block up more quickly, more power and faster flow will push water and finer sediment through the filter to be picked up by the vortex-chamber.

I generally stick to a once every 1 to 3 days cleaning regime, unless I have done some in-pond cleaning which has stirred up a lot of sediment and weed, in which case it may require a couple of times a day for a couple of days. Really its simply a case of watching the flow of water, and getting used to how much muck the filter picks up.

To clean I do the following, and I also give this list to the neighbour if we go on holiday for more than a couple of days:-

White Pre-Filter: Clean once daily.

1. Turn off water control valve.
2. Unscrew top, pull out cap.
3. Position yourself so you won’t get splashed.
4. Turn water valve on - the water pressure will push the filter pads upwards so you can easily grab the handle and lift them out fully, let water run for 5 seconds to let crappy water overflow out of the open top, then turn off the valve again.
5. Rinse out, squeeze, clean little filter pads under running tap. Go outside and gently swing the filter pads in a circle to get rid of excess water (because you don't want chlorinated water going back into the bio-filter). Be careful not to bend the filter pad wire when swinging it!
6. Put filter unit halfway back into white tube. Put cap back on and use this to push down.
7. Screw top back on. Sufficiently water-tight but not too tight.
8. Turn water valve back on, and check flow.

Main Bio-Filter: Clean once weekly, or every couple of days. Although it will probably last 2 weeks if necessary.

1. Get a bucket (if you want, don’t have to – just let the water spill onto the garden, but keep clear to prevent getting splashed).
2. Turn off water control valve (not necessary, but helps in case something goes wrong).
3. Gently unclip big green vertical pipe from clip.
4. Grasp u-bend pipe at bottom firmly.
5. Rotate green pipe forwards and down, making sure not to pull it away from the bio filter otherwise the pipe might come out. Not a problem if you do it slowly and carefully. (This has only ever happened to me once).
6. Hold bucket in place. Rotate down and over bucket, let run to get rid of dark gloopy water, then carefully rotate back up.
7. Clip back into place.
8. Turn water back on, and check bio-filter fills up and water starts flowing out ok.
9. You’re done. Empty the gloopy water on the tomato plants!

A More Thorough Cleanout:

I have discovered a more thorough cleanout method for the main bio-filter without having to remove the media from the filter. But I only do this once every 3 or 4 months, because it can disturb the bio-filter media a fair bit, but dislodges quite a lot of muck out of the bio-filter. The downside is this can also have a negative effect on the bacteria, which may result in an algal bloom and green water for a while. It also empties a fair amount of water out of the pond which has to be replaced, and if you're using tap water to top up the pond this may also contribute to an algal bloom while the bacteria recovers which could be a few days or even a week or two. I recommend injecting some new bacteria culture into the bio-filter after this clean out to boost the bacteria.

After doing a couple of full empties and re-fills of the bio-filter as described above, except this time completely emptying the bio-filter of all water, then filling right up a couple of times, I then fill it up again and turn off the water flow again.

Now I unscrew the pre-filter top cap, remove the pre-filter media pads assembly, and screw the top back on.

Then I push the pre-filter pads down into the main drain of the bio-filter (the green vertical pipe). This pipe of course is full of water.

I then make a rapid up and down pumping movement of the pre-filter pads in the green pipe, which forces and kind of "shakes" the water right inside the bio-filter.

This helps to loosen sediment trapped in the bio-filter media pads. After a good minute or so of "shaking" I then tip the green pipe over and empty out the water from the bio-filter again. You will be amazed at how much additional silt has been brought out by this method.

I repeat this by filling again, doing the pumping action, then emptying, before finally filling the bio-filter, replacing the pre-filter pads back in the pre-filter and getting everything running again.

Look at that amazing Watercress plant growing out of the top of the bio-filter
and spilling down the sides onto the ground.

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