If your camera is a SLR (with built-in light meter) try and use it without the flash. If your light meter says it is bright enough, then you should be fine. If that's not the case, or your camera is a basic point and shoot, try shooting down into the water to avoid the glare from the glass. If you must shoot through the aquarium glass, try to do so at an angle that will miss the glare from the flash. Tricky, and you may waste film.
The only good shots I have gotten from my tank are ones without flash using natural tank lighting. I had to crank the exposure way down but they were fine as long as I used a tripod.
Sorry if that doesn't help.