I may have just figured out a way of lowering nitrates in my tank. Having viewed some commercial biological nitrate filters it is clear that to remove nitrates you need anaerobic bacteria. With a normal external canister filter that circulates water from the main tank at a relatively high rate you have too much oxygen for the anaerobic bacteria to thrive.
In order to breed the anaerobic bacteria you need a flow rate low enough to guarantee that your canister filter contains oxygen depleted water. The commercial systems use a drippier to feed the filter with drops of water, where the flow rate is 1 - 4 l/hr ( 0,25 - 1 gph) for a small tank (5 L canister) and 12 - 30 Liter/hr ( 3 - 8 gph) for a large tank (30 L canister).
Building a drippier system is complex but putting a timed relay on a normal canister filter isn't. I set a relay timer circuit to 90 seconds off, 2 seconds on. I measured the flow for one pulse to be 100mL, which averages out at roughly 4 liters per hour. I used a spare AVEX 600 external canister filter with a volume of 2.3 Liters. I replaced some of the filter medium with porous clay pebbles to increase the surface are and encourage bacteria growth inside the pebbles where the oxygen will be at its lowest. I used a Velleman adjustable interval timer placed inside a plastic box with a 12V wall mounted power supply. http://www.vellemanprojects.com/be/e...iew/?id=366732
I took some water out of my tank with quite high nitrates and put it in a test tank. I then left in with my anaerobic filter for about 1 month. When i measured the Nitrates they had fallen to zero. I have since connected the filter to my main tank and I'm eagerly awaiting the results.
Has anyone tried this before? Will it work on my main tank? What removed the nitrates from my test tank?