Thanks everybody!
These are shot with a Canon 20D. I shoot on AP or Manual, with a 580 EX on-camera, pointing up at about 45 degrees, and a slave 580EX directly above the tank pointing down, both with Omni-bounces. My ISO is usually at about 200 or 400, depending on the lighting. I'm manually focusing (my 17-85 IS lens' autofocus is out right now).
I usually end up shooting during the day, because my other half comes home at night and he's already grumpy about "ALL" my fish tanks (all two). He's too jealous of the little fishies for any photos at night. If you can shoot at night, it's easier, because you get fewer reflections off the glass. A darkened room always helps.
Oh, and clean glass! I always forget, but if you remember it will be easier than trying to shoot around algae or water spots.