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THe problem with shooting a live fish is the glare off the glass! I had to shoot this tang with a rubber lens hood pressed up against the aquarium to control the glare and reflections. But, if there were a way to do it, I would love to try it out!
Ideally, fish in an aquarium should be lit from above i.e. from inside the lid. If that is not possible, the flash should aimed be from a position about 45 degree to the axis of the camera. Flash from the camera position will always give a sharpish refected image of the flashgun firing, such that masking it out may be essential, although a long enough focal length (telemacro) might work by framing to exclude the reflection.