06-26-2009, 12:37 PM
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The number of fish you can have in any tank is dependant upon several factors, some more important than others but all necessary.
The major issue is bioload, which is the effect each fish has on the environment in a closed system; that is, how much waste does it produce (through respiration and excrement) which interacts with the biological processes; and you must think ahead to the fullgrown adult size. The bacteria have to be in sufficient population to handle the biolode from fish, invertebrates and plants. The gaseous exchange has to be sufficient for the number of fish (and plants). There is also the matter of nitrates produced by these biological processes, and these come into play with the second and third points below.
A second thing that must be in balance with the stocking level is your tank maintenance (and here I include the weekly water changes and the filtration system). If the bacteria are adequate for the bioload, the ammonia converts to nitrite which converts to nitrate; plants can use some of the nitrate, and the rest you remove with partial water changes. Nitrate is also toxic at high levels (as can occur with a greater density of fish) and anaerobic bacteria in the filter and the substrate will break up nitrates to obtain oxygen, creating nitrogen gas in the process. Water changes are extremely important in any aquarium, but even more so the more and/or larger the fish.
Thirdly are plants; a planted tank has a somewhat greater capacity (providing the plants are growing healthy of course and not dwindling away adding to the bioload themselves) because they are nature's "filters" in the aquarium. I was recently reading an article on filtration by Dr. Ted Coletti in the July TFH in which he makes the point that the first line of filtration, and the best overall, in any freshwater tank are live plants; more biological filtration takes place on or through the plants than any filter we could use.
Fourthly are the water parameters; if the pH and hardness and temperature are the optimum (best) for the particular fish in the tank, they will have much less stress and be less susceptible to other factors that would otherwise cause trouble. In other words, the fish in the tank must all require roughly the same type of water with respect to pH, hardness, salinity, and temperature. The benefit of this is that no fish will be constantly using its energy internally to "adjust" to conditions that are outside the norm for that species, and the result is less stress and better health. Fish have a complex internal physiological process that is greatly affected by the water in which they live.
Fifth is compatibility. The fish in an aquarium must all share behaviours that are compatible and do not lead to continual bullying or aggression. Fish that are constantly, or largely, under threat from bullys and aggressive fish will be more stressed and suffer poor health leading to disease. [I am not referring here to normal spawning behaviours between fish of the same species.]
Each of these aspects must be balanced and in sync to avoid problems. At this point I should also say that the experience of the fishkeeper has a lot to do with it. First by selecting the fish wisely so the above criteria is met, and second by being prepared should something go wrong.
You want to ensure you don't overload the system. In the April 2008 issue of Aquarium Fish International, there is an article on setting up a biotope of the Orinoco River in South America using a tank 30x15x18 inches, which if memory serves me is about 40 gallons. The author, Oliver Lucanus, suggests 118 fish in this setup. In his description of a suitable biotope aquarium for neons or cardinals, Heiko Bleher suggests stocking a 40g aquarium with approximately the same number of fish. In my own experience, I had more than 130 fish in my 90g for several years, and presently there are 112 fish in this aquarium. In all these examples, we are talking small adult fish, less than 2 inches and many only 1 inch, that are compatible (same water requirements and behaviours) being maintained by relatively experienced aquarists. Fifty neons in a 55g would work fine; 50 angels would be a recipe for disaster and the fish would not be in good health and probably not survive [I purposely use survive, not live] for long. Fish that are "crowded" develop internal problems related to "stunting" and have difficulty particularly with their immune system.