You can't get it right on the first shot, so when you commit to a photo session, plan on shooting at least an entire roll of 24 or 36 exposures. If you're like me, you'll shoot an initial roll to test the setup and then come back to shoot a few more rolls after seeing the first results. Setting your lights can be tricky, but it's really the key to getting good pictures. There are several reasons. The aquarium light may be the wrong color temperature for the film you are using.
Photographic lighting is matched to color films.
You will get better results by using a finer grain film. I highly recommend that you get the flash unit off of and away from the camera for this type of photography. Because of the reflection problems created when shooting through glass, the light needs to enter the tank from the side, from above or at an angle to the camera's lens. In other words, light will reflect off the glass at the same angle that it hits the glass. If this seems too technical, simply avoid aiming the flash directly at the aquarium with the flash mounted on the camera.