Here another idea. How about having, on a long random timer, a quick cat's paw dropping down into the tank from the top, a single splash (with sound effect?)with the fish doing a quick dart away from it. It would probably have to be a 2d paw, but if it's a fast in and out maybe people won't notice.
Ascii
p.s. Forgive the image to the left, I usually post on the Everquest forum and that's my character on Everquest (well, ONE of them)