Ahh the frustration, and satisfaction of shooting an aquarium. Well, I have a 29 gallon aquarium, and have trouble shooting it as well. What I have learned is that is to best to have all other lights off other than the tank light. Night time is best if there are windows in the room, and you wont have to worry about any glare, unless you are using flash.
Also, try setting up the camera on a tripod or something in front of the tank, so the fish kinda get used to it, and after a while it will help make it easier to capture them.
You may have to raise your ISO or something like what was stated above, and after some practice you should get it.
Here's a couple of my shots, as you can see, I'm still learning as well
