[vertex]

I'd use a tripod and then put it on a timer so you have time to let go of the camera so it is nice and stable before taking the shot.

Put all your lights on the tank ON. But darken the room as much as possible so you don't get any reflections.

If you can control the exposure, you may need to use a faster film setting like ISO200 or even ISO400 so the fish aren't blurry with a long shutter. Keep in mind higher ISO gets grainier though, so it takes some test shots to see what works best.

Also, you should play with any color or white balance settings you might have. I find that I get the best color settings by doing a custom white balance and then shooting with that. If you have vivid color settings or daylight or other things, try one each and compare the results.

BTW: You can see my tank pics using this technique in my signature (33 G reef pics link).