Tank Cycling: A simple Explaination

(by luvfishes Sept 10,2003)

Fish give off ammonia, thru respiration. Decaying food and solid poo also decompose into ammonia. Ammonia is deadly to fish in small amounts. Bacteria #1 are attracted to the ammonia, and start eating it. They give off nitrITE which is also toxic at low levels. Bacteria #2 then come along and eat the nitrite, giving off nitrATE. NitrATE is not nearly as toxic as the ammonia and nitrITE, but it does accumulate, and needs to be removed via those partial waterchanges we do every week.

Now, these little bacterias need a place to live. While they will gladly live on solid surfaces in the tank (rocks, decorations, etc), their preferred home is in moving water with lots of Oxygen. That would be the filter in a fishtank.

These little bacteria are the reason you very very very rarely ever completely dismantle and "clean" the decor in an established tank. They are also the reason we advise to NOT throw out the filter stuff, but to rather rinse the floss or foam in "used" tankwater that you've removed when doing maintenance (partial water change). Hot, Cold, Chlorine and some medications can and will kill off our friendly bacteria. We need to treat them with some care if we want the fishies to be happy and healthy.

So that's the short and sweet version of what goes on. There are a few ways to make this happen in a new tank, from adding ammonia from a bottle (NO FISH!!!) to adding a couple of hardy fish and testing like mad and changing water when needed - ie the ammonia and/or nitrITES get over 0.5-1.0 ppm

There is also a new product out by Marineland, called BioSpira, which adds the necessary bacteria right from the beginning.

IMO all other "bacteria in a bottle" are a waste of money, so you don't need them. Spend the money on tests for ammonia, nitrITES, nitrATES and pH, instead.


RED denotes a very toxic substance. Keep the level below 1.0ppm in a new, cycling tank.

GREEN denotes a less toxic, but still problematic substance. Keep the level below 20 ppm for most fish.

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