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. A flash is necessary. Do not aim floodlights or any other bright household lights at the fish. This may allow enough light for using 400 or 800 speed films but the results will be a yellowish blurry image.
Most flashes generate a quick burst of light at a certain color temperature.
The quantity, however, depends on the size and model of the flash. I have seen advanced fish photographers use a single, immense flash, powerful enough to illuminate an object 50 feet away. I never use a single source of light. I use two or three sources. I have my cheap flash attached to my camera by a connecting cord.