As a photographer, I discourage using flash at ALL. Especially with photographing fish. You will always get that awful glare from the tank glass.
I can imagine that it'll scare your fish too.
:) If you know how to using the settings on your camera, I suggest turning off the flash and use Macro mode. And if you REALLY know how to use your camera, use the smallest f-stop number for the shallowest depth of field (and to let in more light), an ISO of maybe 1000, and a fast shutter speed (enough to catch quick fish, but slow enough so that it doesn't mess with lighting).