Shoot at night. Use a tripod, the back of a chair, or SOMETHING to hold your camera perfectly still and aligned with the vertical and horizontal centers of your tank. Turn off the flash and all lights in the room. Use the delay on your shutter release so you don't cause blur when pushing the button. If you're having difficulty focusing, try locking the focus on an object away from the tank equal to the distance between the lens and the back 1/3 or so of your tank. If you have different ambient light settings on your camera, play around with them to see if they capture better color. Good luck!