Use the Pond – Nitrite Test to test for the amount of Nitrite in the water column. Nitrite is produced by beneficial filtration bacteria as they break down the harmful waste ammonia excreted by the fish. Like ammonia, it is not visible to the naked eye. Although it is not as toxic as ammonia, nitrite is still very damaging to the health of the fish. Nitrite itself is broken down into nitrate, which is practically harmless to fish.