A couple hints for good fish pics:
-Lots of light. If you have multiple tanks, put as many lights as you can on top of the one you're photographing. Give the fish some time to adjust, so they get back to their normal selves.
-Fast film, or if digital, high ISO setting. I use ISO800 film in my 35mm camera. The digital only goes up to ISO400.
-Wide aperture, for two reasons. First, you let more light in, allowing a faster shutter speed. Also, you get a more narrow depth of field, meaning a sharp, in-focus fish and a blurry background. Makes the fish stand out more.
-Fast shutter. Well, as fast as you can really get away with. That'll cut down on motion blur.
-Clean glass. I can't stress this enough. Dirty glass makes the photos blurry and cloudy, not to mention it messes with auto-focus.
-Patience. Expect to spend a good hour in front of your tank with a camera. Be patient and wait for the fish to come to you before taking the shot. Pick a good scenic part of the tank and just wait for the fish to come in front of the lens. It'll happen.
-Shoot lots. Your shot ratio in an aquarium is going to be absolutely atrocious. I take hundreds of photos, only to end up with one or two that I like. It's just a fact of life when you've got low light and motion.
If you're working with an SLR system, you probably don't need this advice, but it's something i've found. Use a long, prime lens and sit back from the tank. The prime lens will give you a big aperture and the long focal length will give you a very narrow DOF. The resulting photos are great. Some of my best came from my 35mm SLR. Unfortunately, it's very expensive when you've got whole rolls of film with no good shots.
Oh, and thanks everyone for the kind words about my discus. He (she?) really is my pride and joy. Such a curious, active, and personable fish.