I wrote out a whole guide but i have no idea what thread it was under, here is what i do.
1st you need to make sure all your lights are off and its dark. Only turn your tank light on, or if you do not have a tank light just get a desk lamp over the bowl or whatever.
Next you need to make sure you put your camera on Manual Focus. You then have to zoom in and out with the focus, not the actual zoom (dont ever use that). You may have to have the camera on the glass or a foot or so away, depending on how wide and deep your tank is and where the fish is located.
Next Put your ISO to the highest it can go, on my little point and shoot i put it on 800.
Then youll need to adjust your white balance. You can get a completely different shade, or color depending on which one you choose. Go though them all and pick which one you like best, it really depends on the color of the fish i think, or the color or tint of your light bulb.
After that your pretty much set up. I angle the camera up and down usually so that i dont get the direct glare or reflection of my fingers or the camera it self.
DO NOT USE FLASH!
Good luck :D Sorry if i left anything out. i wrote that fairly quickly.