In all aquarium systems, biological activity develops in your filter, and on the surface of gravels and decorations etc. These bacteria are essential to achieve a balance in your aquarium and different types of bacteria need to be present in the right proportions to break down waste and deal with ammonia, nitrite and nitrate, which can all be harmful to your fish. Every time you change or add something in your tank, or you clean it, you potentially upset the balance because you are effectively removing some of these bacteria.
The replacement of plants and coral or cleaning of filters, gravel or substrate, even in part, can also cause a NITRITE condition to re-occur, even in wellestablished aquariums. Complete sterilisation during the cleaning of an aquarium will also result in a NITRITE condition because you have removed bacteria in the sterilisation process.
NITRITE is toxic to fish, even in quantities as small as 1ppm. If you are starting a new aquarium, it is recommended that you don’t put new fish into the system until a NITRITE reading of less than 1ppm is achieved (ideally zero).

RECURRING NITRITE CONDITION
The biological activity in a filter may take as long as three months to develop fully, and in this time, due to increases in the number of fish, the NITRITE condition may re-occur. Evidence has shown, by lowering the pH to 7.5, the toxicity of NITRITE is minimised. If a system does develop nitrite on a continual basis, after it is fully conditioned, it may mean that the surface area of the biological filter is undersized, or the number of fish in the tank is too great.
ASSISTING BIOLOGICAL ESTABLISHMENT
To assist, ensure the temperature is steady, at or above 26 deg C, check the flow of water through the filters is well distributed and sufficient, and oxygen levels are adequate. When cleaning your filter media, use cold or lukewarm water only as hot water will kill the bacteria.