Freshwater Aquarium Carrying Capacity
From Microcosm Aquarium Explorer
How Many Fish Will Your Tank Support?
There are a number of factors that will determine the carrying capacity of your aquarium.
If you overcrowd your tank, water quality will deteriorate rapidly (e.g., declining pH, nitrate buildup, low oxygen levels). There is also a greater likelihood of behavioral problems, namely aggression.
Poor water quality and antagonism increase fish stress levels which make your aquarium charges more susceptible to disease, listlessness, color loss and other ailments. Therefore, it is always better to have one or two fish too few, than one or two fish too many!
It is hard to know exactly how many fish an aquarium can sustain. The most common rule of thumb for a well-maintained, properly set-up, tropical fish aquarium is one inch of fish per gallon of tank. (These inch-per-gallon equations have inherent problems, but are “aquarist-friendly” and typically give us a good starting point.)
It is very important to reach the carrying capacity very slowly. You should add approximately 20% of the tank’s carrying capacity in fish in the first week. For example, if you have a 20-gallon tank, add 4 inches of fish. Wait several more weeks before adding more fish—then you can add another 20% of the tank’s carrying capacity (that, of course, would be another 4 inches in our 20-gallon aquarium).
Continue to do this ever couple of weeks until you reach about 80 to 90% of the carrying capacity (in our 20-gallon tank that would be 16 to 18 inches of fish).
With luck, the extra 10 to 20% will cover future growth.
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Excerpt from: 101 Best Tropical Fishes