Try using the flash and holding the camera at an angle so that the flash doesn't glare off of the tank. Wait for the right moment when the fish isn't moving(try taking a pencil and pressing its eraser against the glass to get his attention...normally they will pause to stare at it)
...Oh, and try zooming in on the picture once you've taken it. I've found that some picture's that look blurry at first are actually pretty good once you get a closer look at them.

...I think turning off the room lights also helps a bit as well...