Set up your camera on a tripod and prefocus on one particular part of the aquarium such as a leaf on a plant and wait for a fish to swim to that place. Trip the shutter when he is in the right place. For this to work you need to basically You need to find a balance between shutter speed (preferably fast), Aperture (preferably narrow F22) and iso speed (preferably high) and maybe use a flash. This means manual exposure and manual focus (Autofocus is often too slow but sometimes int does work). Sometimes it is hard to find the right balance But when you do you get a great photo.