Thanks famman...but I have to say, Mojo is THE man on taking fish pictures...but here's my tips:
IME, you need to accomplish enough lighting to get good exposure with no flash. (if you use a flash, you'll probably have to adjust the coloring with photoshop, I find it hard to get the exposure just right). I usually do this by doubling or tripling up lights on the tank I'm photographing by borrowing lights from other tanks. Otherwise, table lamps do a decent job if you put them on top of the tank. You want to make sure you don't get a glare from any lights behind you...they end up like terrible "sun spots".
Other tips:
Look for high contrast zones to take the picture. In other words, if your bright colored, try a darker background, and vice versa.
I have to switch to manual focus because my camera takes way too long to AF on a quick moving fish.
Use macro if you can.
Sit right in front of the tank and expect to be there a while (1/2 the time waiting for your fish to get used to you with that thing attached to your face). I usually get 1 decent picture per 25 shots taken. Thanks goodness for digicams.
I used all of the above to get this picture, which while being just a guppie, turned out to be one of my favorite pics.