Nitrogen CycleStages
Stage 1
Ammoniais introduced into theaquariumviatropical fishwaste and uneaten food. Thetropical fishwaste and excess food will break down into either ionized ammonium (NH4) or un-ionized ammonia (NH3). Ammonium is not harmful totropical fishbut ammonia is. Whether the material turns into ammonium or ammonia depends on the ph level of the water. If the ph is under 7, you will have ammonium. If the ph is 7 or higher you will have ammonia.

Stage 2
Soon, bacteria called nitrosomonas will develop and they will oxidize the ammonia in the tank, essentially eliminating it. The byproduct of ammonia oxidation isNitrites. So we no longer have ammonia in the tank, but we now have another toxin to deal with - Nitrites. Nitrites are just as toxic totropicalfishas ammonia. If you have a test kit, you should be able to see the nitrite levels rise around the end of the first or second week.

Stage 3
Bacteria called nitrobacter will develop and they will convert the nitrites intonitrates. Nitrates are not as harmful totropical fishas ammonia or nitrites, but nitrate is still harmful in large amounts. The quickest way to rid youraquariumof nitrates is to perform partialwater changes. Once your tank is established you will need to monitor your tank water for high nitrate levels and perform partial water changes as necessary.

There are other methods to control nitrates inaquariumsbesides water changes. For freshwater fish tanks, liveaquariumplants will use up some of the nitrates. In saltwater fish tanks, live rock and deep sand beds can haveanaerobicareas wheredenitrifying bacteriacan breakdown nitrates into harmless nitrogen gas that escapes through the water surface of theaquarium.