I second the comment about white balance. It made a world of difference for me.
Unfortunately, it still was not enough to make for good photos from my Nikon Coolpix 2000. Thank god Nikon has the best macro mode around, as it's the only thing that saves that camera from being toss IN the aquarium.
Kracker:
ISO Speed is how long the camera exposes the picture. The higher the number, the faster the shutter speed, which means you can catch faster motion without blurring. Unfortunately, higher numbers also make for grainier photos and less depth of field, so it's a trade-off.
Macro is a setting the sets the focal distance to within inches of the camera, at the expense of depth of field. You can take incredibly crisp, clear close-up pictures, but anything more than a few inches away will be blurry.
In the aquarium, I tend to take most pictures in macro mode, except for the critters towards the back, in which case I have to use normal modes. But like I said, my camera sucks for aquariums, aside from macro mode.
And on that note, here are a couple shots I took the other day.
A macro shot of torch coral tentacles:
A normal shot of the torch from a distance (zoomed and cropped), using a high ISO to "freeze" its rapidly swaying tentacles.
Unfortuantely, I deleted a similar picture taken at low ISO, in which the tentacles came out really blurry from the motion. But here's a normal shot of a BTA w/ clown, taken at a low ISO. Note how blurry the clown is because of the high exposure time.