Sponge filters are not as bad as people make out, it is better to have a sponge filter than not because they are perfect habitats for aerobic bacteria which breakdown ammonia and nitrite which are much more poisonous than nitrate to corals and fish. their end product is nitrate which requires a different sort of filtration, but if your tank has had fish in it for a while and the ammonia level and nitrite level are low then the sponge part of the filter is not needed. Nitrate is broken down by anaerobic bacteria, if you have live rock in your aquaria or a DSB then this process is already there. You can buy anaerobic filters which will breakdown nitrates, all they do is slowly drip water through their media which could also be a sponge and convert nitrate to nitrogen. This is actually a naturally occuring process in the aquaria if you have rocks or sand with rocks placed on top of it, you already have these oxygen depleted zones. The only difference is that the nitrate filters that you can buy are dosed or require dosing with a carbon source. You already have anaerobic zones in the aquaria all that is required is adding the correct dose and skiming of the byproducts and excess bacteria. It is vital that the aquarium is protein skimmed constantly during the first month of dosing; especially if the aquaria has a high amount of nitrate or phosphates. Sugar or alcohol (vodka) can be dosed into the aquarium (I recommend very low levels) especially if the nitrate or phosphate level is high; nitrate >5mg/L Phosphate >2.5mg/L. Dosed weekly, with a few grains of sugar at a time, slowly building up to about half a tea spoon will cause a rapid increase in anaerobic bacteria breaking down the sugar and the nitrate, in this process they also use up phosphate. A lot of by-products will be produced which can be protein skimmed off and removed from the system. I have dosed this in my own system with reduced nitrates and phosphates, the key is building up the dose even if you have very high phosphates and nitrates, otherwise you will end up with bacteria on the surface of your aquaria and a very cloudy tank. Phophate media and phosphate filters use iron in anaerobic conditions to reduce phosphate, the iron bacteria breakdown the iron by using phosphate instead of oxygen.
