I think one important part of getting real good pics is to get a tripod (or something to keep the camera super still). Then turn the flash off and get a bunch of extra lighting over the tank if there isn't enough to lighten the image up already. Keep the lighting over the very front of the tank and then just take a zillion shots. If you take enough you will eventually get a good one. It takes a while though, especially with quick moving fish. For those you may need more light and then quicken the shudder speed so you don't get a blur.
The key is patience. You will also be limited by the capabilities of your camera. I know I need a new one. I'm by no means an expert though. I'm sure someone who knows more about photography will know more than that and can help you out more.