Also if you're going to use flash closer to the tank, you can angle your camera so the flash is out of the shot. Example from tonight (although the flash is seen in the corner, it's not in the way of the subject):
^^I'd recommend not doing that a whole lot. My fish don't react much to it, but others might.
of course without flash you can get a totally different effect. here's the same fish.
(with a slight blur because I shake)
Working on my digital photography skills, but this is what I've gotten from it so far.