As stated, it really depends. Are you shooting moving fish or stationary coral? Really there are no settings that will universally work. Your glass may be thicker, light may be brighter, etc....
Definitely do some reading on DSLR settings, it all applies here.
Basically it's all a balance of the 3 main settings, shutter, aperture, iso.
You usually use the slowest shutter speed you can use without blurring due to less light reaching the lens on a reef tank.
Use the aperture to adjust the depth of field to an acceptable range. Larger aperture number means greater depth of field, but less light reaching the lens.
ISO is like a last resort to boost lighting, but it also adds noise.
Shooting a moving fish would require a higher shutter speed, then balancing the aperture and ISO to get a decent image without blurring.
Shooting a coral with a macro lens would likely be shot with a slow shutter speed on a tripod, adjusting the aperture to get a decent in focus range, then adjusting iso to set the brightness. You may have to lower the aperture to get a result without noise in some instances.
Read some general photography tutorials, you'll get it.