What kind of camera/equipment do you have? Generally have the tank light on and room lights off to prevent glare. If you have a removable flash with a cord or wireless remote a great way to shoot is to put the flash over the tank firing down into the water over the fish, or even hold the flash off of the camera off to the side pointing toward the fish at an angle. These are all things that will help put more light on the fish. If you have a shop light that you can hang over the tank to throw light down into it, that may help as well.
If you don't have a removeable flash, you can sometimes get away with using the on camera flash of you shoot from below the fish aiming up at a good angle. Even if the flash shows up as a glare in the glass, it will often be far enough away from your main subject that you can crop it out in post processing.