My approach to photographing fish in aquariums is to use a lens that can be placed directly on the glass, then use a flash to direct light at the fish. This keeps the focal plane parallel to the air/glass and glass/water boundaries, so you don’t get odd refraction ruining the shot. It also blocks extraneous light from bouncing off either of those boundaries and into the lens, where it could cause glare that also ruins the shot.

However, if the fish is really shiny, even this approach doesn’t give you a great shot as it can easily be over- and under-exposed in the same image. I typically aim for under-exposure in this situation as, while not as accurate as I’d like, it does create a rather striking image.