The two most critical factors responsible for exceptional aquarium photography are the two 'L's - lens and lighting. Of course, a dirty glass can ruin a good picture but with good lighting and a good lens, the fish behind the dirty glass will still be crisp and clear. This is a brief explanation of how I use light to photograph my fish.
Most photographers have their own techniques for everything.
Whatever works well for you is the right technique. The light from the hood or canopy and the ambient light simply is not enough to sufficiently illuminate the fish. A flash is necessary. Regardless of your technique, I must mention that if you are relying on just the dim fluorescent lighting of the aquarium hood for light, you are going to be disappointed. The light from the hood or canopy and the ambient light simply is not enough to sufficiently illuminate the fish.