#1 - Patience...you'll need a lot of this! Fish don't like to pose.
#2 - Use the largest aperture setting you can get...you can re-size later
#3 - You need something to keep your camera steady. A tri-pod is preferred.
#4 - Use macro and auto-focus settings.
#5 - Using the flash is what causes the reflection...the flash helps to quicken the shutter speed. You can use flash if you angle the camera slightly in the opposite direction, never directly head on.
#6 - Turn off all room lights.
Experimentation with your camera and subject will allow you to choose which is best for you. I have used both flash and non-flash to get pictures of my fish. Angels are difficult because they won't cooperate and stay at the top (which is the darkest part of the tank), so I use the flash for them.