Minimum # of fish - Tropical Fish Keeping - Aquarium fish care and resources
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post #1 of 4 Old 03-01-2011, 11:02 AM Thread Starter
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Minimum # of fish

Here is a wtist on the usual question " how many fish can I keep in a 55"
I want to know what the minimum number is to keep the tank cycle going.
I have transfered my fish to my 125, but want to keep the cycle stable.
I have 3 corys, and 3 giant danios in there, but want to rehome the danios.
I am running an eheim canister.
Will the 3 corys be enough to keep everything happy???
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post #2 of 4 Old 03-01-2011, 11:07 AM
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Yes. Small numbers are great. I had a female betta in a 40 gallon tank and it was wonderful. You tank will readjust itself for the lighter load.
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post #3 of 4 Old 03-01-2011, 11:22 AM
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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.


Byron Hosking, BMus, MA
Vancouver, BC, Canada

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian. [unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]
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post #4 of 4 Old 03-02-2011, 12:12 AM Thread Starter
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Thanks.... I am not sure what I am going to do with this tank yet, but I want to keep it cycled and ready.
We are actually talking about turning this into my first saltwater tank.... if the budget will allow it.
I have the feeling that it is gonna stay fresh though
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