i shoot with a D40. i shoot mostly in S mode. here's what i do to get really nice shots:
- add extra lights on the tank. this is KEY. you needs lots of light unless you running a wireless flash and/or high speed lenses. T5HO fixtures come in very handy for this.
- shoot in S mode
- take off auto-ISO and set ISO to 200. a lower ISO will give you better quality pics. that is why you need lots of light.
- adjust your shutter speed to give you a focused and well exposed shot. for me this is usually between 1/40 to 1/100, depending on how close the fish is too the light. the closer to the light and the brighter the fish looks, the faster the shutter speed i use. the farther away, the slower the speed to capture more light. be careful shooting below 1/30. any slight movement will cause blur in your pic.
- post processing, this is just as important as taking pics themselves. most times youll need to adjust individual color channels to get the proper white balance. you can also adjust exposure, highlights, and shadows if needed.
good luck!