Aquarium Water Chemistry and the Nitrogen Cycle

Fish produce ammonia, which is highly toxic. One group of bacteria converts ammonia to nitrite, also toxic; a second converts nitrite to nitrate, which is far less harmful and removed by water changes and plants.

Establishing those bacterial colonies — cycling — takes four to eight weeks in a new tank. Adding fish before the cycle is complete causes the gill and skin damage commonly called new tank syndrome.

Add a pure ammonia source to the running tank and test daily. The tank is cycled when it can process a measured dose of ammonia to zero, with nitrite also at zero, within 24 hours. Then perform a large water change and stock gradually.

A liquid test kit is far more accurate than paper strips.

Change 20–30% of the water weekly with a dechlorinator, vacuum the substrate, and rinse filter media in removed tank water — never tap water, which kills the bacteria you spent weeks growing.