Use a tripod, tripod, tripod! Then you can get away with no flash if your fish aren't moving too fast. Yes, do a slight angle to avoid reflections. BUT BUT BUT... the worst part about tank photography I've seen has nothing to do with photography. Pick up that dirty laundry in the shot, remove the magnetic algae scraper that's in my way, not to mention that thermometer, your tupperware of fish food selection is distracting & ugly, does your kid never pick up his toys? The wall behind the tank need a coat of paint?.. crop it out of the photo. Think about what you're showing the world vs. what they want to see.