Usually, Macro mode is used for taking close-up pictures, otherwise just the normal "auto" mode for the full tank shots. The most important things you can do is make sure there is a LOT of room light, but no reflections of the light off of the tank. Also, a steady hand is required to keep the picture from bluring.
I've found that flashes don't give you better pictures. Finally - every brand of camera is different, and you will probably have to keep playing around until you find a method that works for yours. The good news is that it's cheap to experiment.