i found this...
" To take good interior shots of an aquarium, you will probably need to push the camera's ISO to its limits. While you ordinarily want to use your camera at its lowest ISO setting, using a setting of 400 or higher is a good idea in this situation because it helps you freeze the action. (When you don't freeze the action, you end up with a blur.) The ISO control makes the camera more sensitive to light and therefore enables you to use a faster shutter speed.
However, high ISO levels also add some digital noise to the picture--random bits of color that don't really belong. The tradeoff is definitely worth it when you're shooting in low-light conditions. Remember, though, to return your camera to its lowest ISO setting when you're done with the fish.
This probably goes without saying, but remember to turn off your camera's automatic flash. If the flash fires, the light will reflect off the aquarium's glass and obliterate the detail in your picture. With aquarium photography, flash is a definite no-no!"