I think that lighting is key to getting good shots. I hate to use the flash because it washes out the colors of both the fish and the tank. I turn all of the lights in the house off, so that just the tank light is on. Then I turn the flash off on the camera. Pictures turn out really crisp and bright, which reflects exactly how the tank looks in person.
It also helps to use a tripod because then you don't have to worry about your arms shaking or anything.