People are often confused by the inexacted meaning of what "cycling" a tank means. Generally it is used to refer to the establishment of the nitrogen molecule metabolizing bacteria. This is just one step of the process of species colonization that occurs when you set up a new tank. Sometimes "cycling" is used to refer to the entire process of species colonization.
A few of the species that you put into the tank are deliberate, most of them either find it on their own or hitchhike with the ones that you wanted. Commonly we add fish, plants and snails on purpose (not always with the snails). Many other species come along including, bacteria, fungus, algae, protists, rotifers, nematodes, microscopic crustaceans, etc.
With each addition of a new species, the minature ecosystem has to readjust and balance. Algae and bacterial blooms are very common during the early stages. These can cause large fluctuations in water conditions in a very short amount of time. Sensitive fish (otos, rams, etc) cannot adjust to these changing conditions very well and grow ill and die.
