My suggestion is to either look into a polarizing filter and try to cut down on the reflection this way, or cheat and put the camera close enough to the glass that you are not looking through the reflection.
The tactics in that link are a little extreme, but the science is the same even with a single polarizing filter and shooting at an angle.
Polarizing filters are very inexpensive, and even if you can't find one for your camera, you really just have to find one that you can hold infront of your camera's lense.
I used the latter method on a bus tour. Basically just put your camera right on the glass. The flash will reflect above the lense (with SLRs anyway). The only drawback will be the potential for reflection on the back of the tank. As long as there is something between you and the back wall though, it shouldn't be too terrible.