The aperture setting (f-stop), shutter speed and ISO will all be dependant on your lighting, area in tank being shot, brightness of fish or coral, speed fish is moving...etc. So many different variables need to be taken into effect. With fish, you need a fast shutter speed, to avoid blur. With a faster shutter speed, you need a wider aperture setting (lower F-stop) in order to still get adequate lighting. None of these settings will effect white balance.
Your camera will have a graph to help you determine if you're getting the proper amount of available light. Usually, the center of the line is optimal "exposure." If you're exposure is to the left of center, you need more light, so you'd need to slow down your shutter, open up your aperture or raise your ISO setting. The opposite steps would decrease available light, if you were to right of center. The use of the Histogram will also give you a lot of available exposure information.
The key is to learn how Aperture, Shutter Speed and ISO all work, in relationship to one another and to control amount of light and depth of field. After learning what all these settings mean, it's a matter of "fiddling" with your camera, taking pictures with lots of different adjustments, and seeing how those adjustments effect the final outcome.
