DIY Koi Pond Filter
Here is a koi pond filter that I built as a final polisher to take out any fine particles that the main filter missed. It's not the most pretty of things or perfect in anyway but has worked well on my quarantine system for over 10 years. This type of filter could be used as a supplementary biological filter and it adds extra air to the water, but would be better modified for this purpose. It's basically a coldwater header tank with a few bits of pipe from the local plumbers and 4 foam filter pads from a garden centre that sold koi supplies. The hollow filter pads were originally sold for a Cyprio filter but they could quite easily be made from foam sheet glued together. Normally it has a lid to keep dirt and light out but I had recently cleaned it out before these photos were taken.
The water is pump fed in through a pipe at the top on the left of the picture. You can see with three of the foam pads removed the water exits via pipes at the bottom of the filter by gravity after passing through the foam.
When clean the water level in the filter stays quite low but as the foam gathers dirt it rises up. The increase in pressure and relatively clean water stops the level from reaching the top but for safety an overflow should be added especially if used on a murky pond.
With the water level low it aerates the water well but obviously the available filter surface area is reduced being out of the water so any biological activity is not to be relied upon. This could be improved and I did think to put some Kaldnes K1 in there but decided this would complicate cleaning it out. As intended for mechanical filtration it works well and gets better as it gets dirty. The filter can be cleaned by reversing the water flow through the filter, back flushing the dirt off to waste but it's just as easy to wash the foam pads by hand in a bucket of pond water.
Building your own koi pond filter can be satisfying and a number of people talk about doing it but you should know what you are doing as it can be a matter of trial and error. The error could cause the loss of your koi and then you'll wish you had bought that professional built system. I have found it's best not to economize on koi pond filtration as it's the life support system for the pond and the basis for keeping healthy koi but if you stick to well trusted methods it cannot hurt to try to make improvements.