Well you want to use a flash definitely unless your aquarium is ungodly bright. Even with decently fast glass, getting a high enough shutter speed to stop motion is hard.
Also, set your white balance to tungsten and use a CTO gel on the flash to correct correct the exposure. A CTO diffuser such a sto-fen will work too.
These are a few sample images i took few months ago with that technique. I'm planning to head back to the aquarium again tomorrow (before my annual membership runs out.)
Edit: you'll want to set the flash at a 45° angle and have the lens pointed straight at the subject to prevent distortion.


This post was edited by Eek on Apr 23 2011 06:21pm