04-01-2009, 07:10 PM
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Cody's comment is important. 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 (unless you intend on giving them away before then). The bacteria have to be in sufficient population to handle the biolode from fish, invertebrates and plants.
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 larger the fish.
Thirdly 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.
Fourthly 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.
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. Things can go wrong for all of us, and when it does, you have to be ready with the knowledge to rectify the problem or lose your fish. I remember my first 5g aquarium when I was in my teens (a long time ago, I assure you!) and how many fish I lost because I didn't know what I was doing and in those days we didn't have the internet for great forums like this one to gain knowledge and benefit from other's experiences and knowledge. I am still learning from articles, books and posts from others on this forum. I rarely lose fish now except to old age. We were all a "newbie" at the beginning, and each experience along the way adds to one's knowledge and the hobby becomes even more fun and very rewarding.
You want to ensure you don't overload the system. Twenty tetras (2 inch max size when fully grown adults) in a 20 gallon is fine, if it's planted, gently filtered (water circulating) and weekly partial water changes of 30-40%. Twenty angelfish in the same tank would be asking for trouble and certainly not advisable. 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 I intend to raise the existing 70 fish to this number if I can ever find the fish I want. In all these examples, we are talking small adult fish being maintained by relatively experienced aquarists. MBilyeu is absolutely correct that clown loaches should be in a group. But, clown loaches can grow to 12 inches, and a group of several of those as adults in a 85g tank (with or without the other fish) would in my view be pushing the extreme too far. However, they are not a problem as long as they are 2-3 inches, but before going down this road you must recognize where it will lead and be prepared.