Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Adding protein (fish food) to the tank adds nitrogen to the whole system.
It does not matter what bacteria or other microorganisms you add to 'clean' the tank, the nitrogen is still there.
Nitrogen can be incorporated into plants, then you harvest the plants (or at least remove dying leaves) and this removes nitrogen from the tank.
Certain microorganisms turn nitrate back into nitrogen gas, and this escapes into the air. But these are anaerobic organisms, and too many of them make the tank toxic.
The best way to remove nitrogen is as I suggested above:
1) Less import. Less fish food. Either because you have less fish, or because you simply stop feeding them so much.
2) More export: Do more water changes. Improve the growth of the plants, then be very diligent about trimming, especially weak or dying tissue.
3) Dilute the problem by adding more gallons of water. New tank, larger tank, sell or give away fish to other people's tanks.
A larger tank all by itself is not the total answer. The nitrate will still rise in this tank, too. Just not as fast, so you might be able to go longer between water changes without the NO3 getting so high. Just as I suggested above: Keep on testing and do a water change every time the NO3 hits 20 ppm.