Photographing fish is quite easy as long as you follow a few golden rules. This part of the site is a guide to photographing tank fish in their normal environment - swimming around a tank. If you have not yet devised your own successful system of photography then you may well find that this section gets you results.
So there you have it. Use a camera with as many manual functions as possible; preferably an SLR. Use a flash, which must be at an angle to, or right up against, the tank glass. Use a small aperture. Acclimatise the fish to the equipment. Oh, yes, and be patient!