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How To Build a Cheap Koi Filter for an Ornamental Fish Pond

Updated on September 28, 2012

Why Filter Koi Ponds At All?

Most koi fish ponds have extensive filtration fitted whereas most goldfish ponds (even of a decent large size) do not. There is no specific reason for this other than koi keepers tend to be more interested in the fish themselves than in the general aspects of keeping a pond. Therefore they generally keep as many fish as they can - which are extremely valuable items - in the accommodation that they have available. Given the value of the fish and the levels of stocking leads to the need to keep the water quality at an optimum and hence the tendency for koi pond to be always filtered.

It is still possible to keep koi in a large natural pond with natural biological filtration in place. It is also possible to filter water for a koi pond cheaply - if you are handy with your hands and reasonably practical. There is no doubt that one of the many expensive koi filters available on the market will perform an admirable job of cleaning your koi pond water - but if you want to save $100s or even $1000s consider a do-it-yourself approach.

A Suitable Koi Filter For a 2000 Gallon Koi Pond

I am going to describe my approach to filtering a 6 feet by 10 feet pond (about 5 feet deep) used to keep koi carp successfully for several years now. The pond was already dug and therefore there was no chance of being able to plumb in a bottom drain or side inlet - everything had to be done from the surface!

My first job was to drain the pond and clean it out. This was some job! Then, after letting the pond dry out thoroughly in the sun for a couple of days I gave the concrete walls two coats of Aquaseal pond sealant to provide a clean and healthy environment for the fish.

I then constructed an outlet about 6 inches below the lip of the pond and cemented a 2 inch elbow into the wall. This pipe, in a culvert along the ground, led directly to the intake of my pond pump some 6 yards away in my garden shed. The other end of the two inch elbow extended as pipe down into the pond. The pump, at ground level, vacuum sucked the water from the bottom of the pond. The pipe entering the pond was surrounded by another 6 inch pipe with holes cut towards the bottom and used as a guard to fend the inquisitive fish away.

I now had a system where I was sucking the pond water from the bottom of the pond into an area of my shed - now I needed some way to filter the water before allowing it to fall back naturally under gravity into the pond.

Buying a good quality filter like this could cost $750 or more
Buying a good quality filter like this could cost $750 or more

The Koi Filter

The koi filter was in my shed and so it was hidden from sight. Nevertheless I wanted to create a neat koi filter that was both secure and I could show my friends!

I bought three old water header tanks from my local plumbing merchants and was fortunate enough to get them at a bargain price as they had been recovered from a building site. I assembled these end to end to form a long line - as long as my shed just over three yards or so. The first tank was set a couple of bricks high from the ground and received the water directly from the outlet of the pump. I installed two pieces of plywood as baffles that caused the water to stop swirling and move slowly over one and then down and under the other. This caused a lot of the debris in the water to settle at the bottom of the tank and could be siphoned out (I have since replaced these with a bank of special koi filter brushes which are much more efficient).

The water overflows out of this firs tank via a couple of tank connectors into the second tank which is filled with "Lytag". Lytag is the pumice type of gravel which is a waste product from power stations that use coal. Unfortunately it is now sold at great expense from aquatic centres along with various other types of koi filter media. You could use $50 worth of graded 20mm gravel from your local builders merchant instead - filling both of the last two water tanks. You could use any filter material you like - I was able to get my Lytag from a nearby power station for next to nothing! Essentially this filter was being built cheaply - and so it may well not be as efficient as many you can buy - but it still works and works very well. I actually do not use anything in my last tank other than old pieces of pipe and odd bits of filter medium left over from my other indoor tanks.

What I have added to my system recently is a UV filter taking part of the outflow under gravity pressure. My koi are stocked pretty heavily - though not as densely as some I have seen - and have always been healthy. It shows that you can keep koi on a tight budget, so long as you keep an eye on what is going on. It is probably the best way to learn about fish craft, about the principles of biological and mechanical filtration - and a great way to enjoy a fascinating hobby!


Saving money is not always the action of a cheapskate! The satisfaction gained by producing something worthwhile, that improves your hobby and...yes...saves you money is only to be applauded.

Biofilters are one item in a very specialist hobby than can be constructed yourself, and will work every bit as efficiently as expensive shop bought items. It pays to do a little research to understand the biology behind the processes going on, as this will help put you on guard for things going wrong and assist you with spotting ways to improve the design and optimize it for your water conditions and stocking profile.


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