In my personal experience with my digital camera, this is what I have done.
Turn all the lights in the house off except for your tank light. Don't stand too close to your tank and I would use either the action or automatic mode to take the shot. 4 out of 5 times I have done this, it has produced some awesome pictures.
I'm sure others have some more technical ways to accomplish this too, but this is the easiest for me. Good luck.