03-01-2011, 12:22 PM
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Once nitrifying bacteria are present in an aquarium, the level will always be what is needed to handle the "food" which of course is ammonia and nitrite. Nitrosomonas and nitrospira bacteria multiply (by binary division, that is, each bacterium divides into two bacteria) if their food increases, but they also die off if it decreases. A tank that has cycled and is biologically stable will have a colony of both bacteria at the number needed to correspond to the fish, plants, feeding (more or less food affects this), etc. Adding to the bioload and increasing the ammonia is usually not a problem since the nitrosomonas bacteria will multiply accordingly, provided the increase is not beyond the capability of the system.
In your case, 3 corys in a 55g will produce very little ammonia, so the bacteria will be very few in number. If you were to suddenly increase the fish load, it might overwhelm the system and cause an ammonia spike which could be detrimental to the fish. Live plants if present will usually assimilate most of this ammonia, which is why cycle issues are (or should be) non-existent in well-planted tanks. Assuming there are no live plants, it is up to the bacteria. Once the nitrosomonas bacteria increase sufficiently to handle the ammonia, then nitrite spikes and nitrospira bacteria must also increase. In optimum conditions, nitrosomonas bacteria require about 9 hours to multiply (once), and nitrospira about 20 hours. Thus, one should never overload the system, as the fish can be negatively affected during these periods.