Look for good glass. I think the key may be flat glass. I had a whole series of shots from The Seas (formerly Living Seas..) that I thought would be great. But the tanks were ALL cylindrical, and I think the curved glass caused significant chromatic abberation (bending of differently colored light to different degrees, causing a sort of slight prism type effect). The only even vaguely sharp shot I got was of the cuttlefish, they had lots of sharp detail.
Sufficient light wasn't the problem. I just couldn't get a sharp image to save my life. The thing was it didn't look bad in person, but for whatever reason the pictures all came out blurry in the end. Either the camera was recording more detail or else something about the optics of a camera lens is more sensitive to this than the human eye.
About flashes.. well, I'd only suggest caution. I've heard that some institutions, aquariums and such, have completely banned photography, or at least all flash photography, because of people who used flashes in situations where they specified no flashes.
I'd have no idea how harsh a flash would appear to an aquarium fish.. but I can only suggest make sure there's no "no flash photography" signs out before breaking out the strobe.
I used flashes quite a bit at my zoo through glass (in places they clearly did not specify no flash photography). None of the situations involved water, they were all air breathing animals behind glass, but the principle should be the same. As has already been mentioned, just don't aim straight at the glass (perpendicular to it) because the flash will bounce back at you.
And Jeff.. nice results overall, but I'm blown away by the seahorse picture. The clownfish picture is good, but the seahorse strikes me as something I'd see in National Geographic.