Most of the problems with tank shots can be solved by putting your camera on a tripod. Then you can fix with settings without the worry of low shutter speeds causing blur. Start with your ISO (if your camera can change it) set as low as possible. Try a shot and see what happens. If you don't like the exposure adjust it accordingly. If when shooting at low ISO the picture comes out blurry due to animals in the tank moving, up the ISO a stop and try again. Repeat this process until you find the settings that work best for you.