I normally open my aperture wide at 3.5 (I don't own any primes yet... no $$ hahaa)
My shutter for fishes is at least 1/80, if not 1/100. Corals can afford to go lower, sometimes even 1/30, but either need to have v. stable hands or tripod. ISO setting normally at 400, sometimes 800. White balance is either at auto or the cloudy day one. I post process my pictures to change the temperature, coz they no matter how i adjust, white balance doesn't come out right all the time.
disclaimer: I never change saturation