The real key is getting enough shutter speed to stop the action for a sharp picture. This usually is acheived by high ISO, larger aperture (2.8 - 4.0) or get enough light
If you have a bright aquarium that helps a lot. Shoot at night and dim all of the other lights around you to minimize reflections.
My tank is rather dim so I'm starting to place a flash in the top of my aquarium and point it up so it reflects back into the tank. I use a remote trigger to set off the flash (on my canon 5d mark II) that way no flash is coming directly from my camera. So far it's working pretty well. I can shoot at f8/ ISO 1250 and 1/100sec (manual mode).
I'm still perfecting my lighting system. During the day you could see thru my tank into the living room. At night it's a cooler perspective.
Here's my Gold Gourami (avatar) shot with this method;