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DIY Pond filters

Ed Seeley

3 Jul 2007
Before telling you how to make your own pond filter you need to work out what you want your pond filter to do and what you want out of it. There are a wealth of different designs you can have, some even ran by no more than a large air pump, but they all work slightly differently and require different approaches.

There are two main ways to DIY a brilliant pond filter that will quite happily filter koi ponds so will be fine for the biggest general pond.

The first and simplest of these is a shower-type filter. These are basically pump-fed trickle filters but on a huge scale! They also only really work well when using a ceramic media called Bakki House Media (BHM) which is very expensive. However when coupled with BHM the results are amazing. I have a DIY shower on my quaratine system and it filters the pool brilliantly. I need to clean it once a year or so and it only needs cleaning this often because this 400g pond (basically a big tank compared to my medium sized, 3,500g main pond) contains 16 koi fed three or more times a day! When properly sized on normally stocked ponds with suitable water flow rates through them (about 3,000gph) these filters never need cleaing! The ceramic media houses so many bacteria in diverse colonies that all waste chucked over them is broken down and processed and large amounts of ammonia is gassed straight off into the atmosphere. All you need to do is water changes weekly or trickle new water in constantly and have old water from the pond overflowing from the pond to water the garden.

So the positives are;
Dead easy to build
Pump fed and so can be sited anywhere above the pond (water needs to fall back in via gravity)
HUGE biological capacity
No regular cleaning
Gases off lots of ammonia so reduces nitrates in the water

Expensive media needed for full benefits
Needs siting above the ground and near the pond so may be an eye-sore
Needs a fairly powerful pump to pump enough water over the media each hour
Doesn't remove fine particles from the water so your water may stay slightly cloudy (but the fish will love it!)
Loses large amounts of heat very quickly so needs enclosing or siting inside

K1 filter
The other easy DIY option is a K1 based filter. K1 is a slightly bouyant, small plastic media for the bacteria to live on. It's great advantages are that it can be fluidised by bubbling air through it. This turns the media over and churns it around meaning particles cannot settle in the media and clog it. The air being bubbled through also provides huge amounts of oxygen to the bacterial colony and will gas off some of the ammonia (but nowhere near as much as the shower type filters). But turn the air off and the K1 floats in a raft and this static K1 is an excellent mechanical filter, removing tiny particles in the water, with the added advantage that to clean it all you need to do is isolate the filter and aerate it, churning the media around and releasing all the dirt.

K1 filters can be either gravity or pump fed so you can use them in almost any arrangement (Gravity is better though as dirt isn't mashed up by a pump before going through the filter!).

Cheap to build from almost any water-tight container
Low running costs (can even be ran completely by air)
Can filter tiny particles from the water giving crystal clear water
Can be sited anywhere and fed by either gravity or pump

K1 is the ultimate escapologist and needs to be contained using strainers and dividers
Static K1 needs regular cleaning as it traps a lot of dirt
Fluidised K1 needs to have enough air in and not be over-loaded or the media won't circulate properly

I made mine with plastic stacking boxes. I cut the bases out of the upper tiers and then used a filter grid cut to size for the new base.

The bottom container has a slot cut in the side so the water flows out like a small waterfall.
Once all the containers are constructed you fill them with media and stack them up!

Then you make a spray bar or two to spray the water evenly over the media. The trick here is to drill the right amount of holes! Too many and the water will all fall out in one spot - too few and you will create more back pressure on the pump reducing the flow. Start with 8mm holes (or thereabouts) spaced evenly across the bar and then drill more if needed. Mine has one every couple of inches and that's a good starting point IMHO.
Obviously you need a decent pump to lift the water that high and remember showers work best by completely ignoring circulation and flow rates. I have a 4,000lph (800gph) pump on my 400g quarantine tank and it's far too weak! I think I need to have 4 times that flow rate for ideal operation! Commercial showers need about 3,000gph flowing over them to work at their peak so don't be afraid to chuck loads of water over them. This is all assuming you are using BHM media though as nothing else seems to work as well (Just a shame it's £140 a box and I needed two boxes in the shower pictured above!).

K1 filter
These can be based around any water tight container basically but the ideal one would be a vortex chamber as it will allow the media to circulate well with no corners to trap in and allow all the dirty water to be removed by a drain. (However water barrels come a very close second and are a lot cheaper to buy!)

As the static filters trap large amounts of waste you need to think about how you're going to get rid of that waste. Believe me this is the most important part of a filter like this. At the height of summer on a heavily stocked pond you may be cleaning this daily and if you have to spend lots of time removing the waste water then you will curse the day you built it each and every day!

Each chamber (including the fluidised one) needs a drain pipe that takes the dirt-laden water either to the sewer (wasteful and not ideal but possibly the only option in a very small garden) or spread onto the garden via a pipe full of large holes (they need to be large so they don't block!). Don't just have all the waste water dumping onto a single spot near your pond or you will have a very soggy area and near a pond that can cause problems with the walls collapsing! Ideally each chamber will slope down to this drain so that when you empty it all the dirty water is removed (This is why I think vortex chambers are ideal - cylindrical and sloped to the drain). However even a flat bottomed chamber will work fine as the air will stir the dirt around so it will almost all go to waste.

You also need to make sure that this drain has a grill to stop the K1 going all over the garden too! K1 will fit through any gap above 8mm. So to make a grill you either need a sheet or pipe full of 8mm holes. You can also use slots and I've used them cut with a circular saw with great success but bear in mind that you need thicker sheets if you cut slots in them to stop them flexing.

The escapology of K1 also means you have to fit strainers on both the inlet and outlet to each chamber. You also need to make sure the outlets especially allow lots of water to flow through them. Basically you can't have too many holes! Remember that in a static K1 chamber all the K1 will collect around the outlet strainer and partically block some of the holes so assume you need twice as many as you think you will and you should be about right! If you're feeding from a static chamber then a 4" pipe with as many holes drilled out as possible running across the chamber will do a great job, or you can have a bend on the pipe with slots or holes cut near the top such as this on my old K1 filter.

The last thing you need to consider is the air supply to the filter. You need air in both the static and fluidised chambers. The fluidised chamber will be on all the time but the air supply to the static chamber will only be used for cleaning. The best way to do this is with Speedfit type plumbing pipe. You can drill 1.5mm or 2mm holes in it to allow the air to release. Again start with fewer than you think you will need and add more to make sure the pump isn't under too much back-pressure.

For a round chamber the air supply is easier and cheaper than rectangular ones like the pictures as you can simply bend the pipe around to form a ring. Site it halfway between the edge and the centre and you will get a strong uplift there and downward circulations at the edges and centre. You can attach the pipe (if you need to as it's pretty strong stuff and fairly rigid) with a couple of blobs of silicone sealer or even better Gold Label Sealant that sticks anything to anything else and even does it underwater!

And get a good pond air pump to supply the air. I have a 100lpm pump on mine but a 60lpm one could do it. There are some good diaphragm pumps on Ebay for less than £50.
Thats some good info there Ed. Can you not incorporate sponges in there for mechanical filtration? I have a 2m x 30cm x 30cm glass tank which i could use and i have a 22,000 LPH pump, what do you think?
Dan Crawford said:
Thats some good info there Ed. Can you not incorporate sponges in there for mechanical filtration? I have a 2m x 30cm x 30cm glass tank which i could use and i have a 22,000 LPH pump, what do you think?

If you want to clear a pond Dan then a K1 filter is the way to go I think. By using a plastic water tank with a strainer to keep the K1 in you can clear the pond in a day or two!

I'm just about to edit the original post with construction ideas! (I posted and then edited so I don't lose all the post if something happens!)
My Koi pond filter is an old wheelie bin, bit of diy and media added, and pondmate pump in' pond. Never an issue. :D