Here's my simple cycling post
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Fish pee and poo and don't eat all of their food. Naturally, all of this stuff decays in the aquarium. One of the things that this stuff decays into is the toxic compound ammonia. Ammonia burns a fish's gills and body. There are bacteria which eat ammonia, but they need some time to build up before they can eat all the ammonia in the aquarium. The bacteria turn the ammonia into a compound called nitrite, which interfers with a fish's ability to breathe, among other things. However, there are bacteria which eat nitrite and turn it into nitrate, which also need time to build up. Nitrate is much less toxic and can be removed with water changes. Ammonia turning into nitrite turning into nitrate is called the nitrogen cycle-hence the term "cycling"(in the process of bacteria building up) or "cycled". When at any point you can test your aquarium and the ammonia and nitrite read 0, the tank is cycled. Nitrate should be kept under 20 ppm. You know that a tank is cycled/is cycling by testing it with a test kit such as the API master test kit. Warning-you need to have ammonia in the tank before a tank starts cycling by definition! A newly set up tank will most likely test perfect because nothing has started the cycle yet!
