We are all professional photographers. Just kidding. Some basic things that may help.
1. Turn off all room lights. This will reduce reflection.
2. Turn on tank lights. Duh.
3. Don't use flash.
4. Sit very still and wait for the picture to come to you. Fish will take pauses and if you're patient, they'll pose once in a while.
5. Use a macro setting on your camera if you have one.
6. Take 300 photos. Keep 15 good ones.