Nitrate Filter

An Easy to Build Nitrate Filter:

by Trevor Pearson

I have been asked many times on how to get rid of the build-up of Nitrates in a Koi pond, especially one that is built indoors..... My Good Friend Peter Waddington told me basically what was required and told me what materials I should use to contruct such a Nitrate filter. That was quite a few years ago many years ago and I having built one and installed it on one of my own systems, which has a volume of 8,600 gallons, I found that although it was very slow to start to make any difference to the readings which we normally had - that after about 3 months is did start to reduce the Nitrate levels in the tank ....

I now have the greatest pleasure of passing that information on to our web site visitors and hope that what I can now suggest works for your own system ... There is no doubt in my own mind that these Nitrate filters work and do reduce the levels of Nitrate within the system.


The main body of the Nitrate Filter is made out of 9" underground waste pipe and has a perforated media support tray which I had plastic welded into place about 6" up from the bottom. Also welded in were the two connections - the inlet which is at the bottom in 1.5" and the outlet in 1/2" which is at the top just above the water level.

Once those welds were made then we had a good strong piece of PVC 1/4" plate welded onto the bottom to act as both a stand and to retain the water in the chamber.

The water is transferred, in our own case from the last biological vortex in the line of 5 back to the first biological vortex via a 1/2" pvc pipe and the pump we used was a 25gph aquarium pump.

The pumping rate should be kept as slow as possible - this allows plenty of dwell time in the filter chamber which is filled with Siporax and Bioholme (sintered glass media) but I am now wondering if using the new Kaldnes (K1) from Evolution Aqua, whether this may do the job equally as well and of course at a much lesser cost than the sintered glass which is quite an expensive media and is also quite hard to obtain these days ..

Finally, I wish you all the best of luck if you fancy having a go at making one of these simple Nitrate filters ..