Q: How many inches of fish could you keep in a 29 gallon tank? Should I use the inch per gallon rule or surface inches rule? Or is there another way?

A: Inch per gallon? *sigh*. somebody please kill this myth. Can a ten inch oscar fit in a ten gallon tank? the number and size of the fish you keep in your tank should be based on several factors. It should not be based solely on the chemistry of your water nor on the magic of the inch per gallon myth. It's more than simply a matter of successfully keeping the ammonia and nitrites at zero or the fact that you may have "great filtration". Certainly these are important issues, but one of the most important factors is almost always overlooked. The number and size of fish that one maintains in a tank should be predicated on the "biology" and behavior of those fish. This means that the interaction of a mixed community tank and/or the behavior of a single species is the single most important factor to consider. You want the fish you're keeping to have the ability to exhibit "normal" and functional behavior. Normal behavior is important. Abnormal behavior results in stress. Stress leads to disease. Disease can lead to death. For example:

1. If species (A) does not get along with species (B), then these fish don't belong in the same tank no matter how few fish you have or how large the tank. "Getting along" is based on the visual and behavioral Q's that each fish species is genetically programmed with. This is why African cichlids don't belong in the same tank with Central American cichlids for example. Africans don't "understand" Central American fish speak and this leads to behavioral problems and stress.
2. If species (A) has specific food or water chemistry requirements which are completely different from species (B), then these fish don't belong in the same tank no matter how large it is.
3. If species (A) is aggressive and defends a territory of 2 square feet, then you cannot expect to successfully keep more than one of these fish in a tank which is smaller than 2 square feet. Convict cichlids are an excellent example of this ... this little fish aggressively defends territories of about two square feet give or take. If you have a tank which allows only that much room and no more, the convict will defend the entire tank to the detriment of any other fish (no matter how large it is) in that tank.
4. If species (A) aggressively defends a territory of one square meter, then you've got to give species (B) room enough to escape beyond one square meter if you're going to keep two different species in that tank. Your tank thus needs to be at least two square meters.

Fish need room to swim without having the rest of the tanks inhabitants "in their face" continuously. When crowded, fish exhibit stress syndromes that result in poor color, improper fin form, insufficient metabolic development, do not exhibit proper musculature, do not develop properly functioning organ systems and most importantly slowly lose their inherent resistance to disease. This results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target.. Think about your local lake or river --- the fish are free to inhabit whatever space suits them biologically. If it gets crowded by their standards, some will disperse and move to other areas where they again have the space they need to exhibit functional behavior. It's difficult to allow for that "space" in your tank -- the fish have no escape within the confines of your tank so it's up to you to insure that the fish have that space in the first place. If your tank "looks bare" .. it's probably just right.

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