Straight on you can't use flash, if you take a pic at an angle so you can see both a side and the front of the aquarium, it might work as the reflection will bounce off at an angle and not back at the camera. Otherwise, to let more light in so you don't need flash and the pic is lighter, try using a higher ISO, slower shutter speed, and the smallest f-stop number your lens can do. Tripods help to keep the camera steady if you need a really slow shutter speed to let the camera capture the light needed. Another thing that might be going wrong is the camera misreading the tank, if it sees the lights of the aquarium as really bright, it might be using settings to make it darker to compensate. If that is the case you'll have to use manual mode or exposure compensation to tell the camera to take a brighter exposure than it thinks is the "right" one.
