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.
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. Fish are acutely uncomfortable if a bright light is shining on them; they often mute down their colors in such situation. Let me repeat this: a flash is absolutely necessary.
Here's the good news: you do not have to buy an expensive flash. Most flashes generate a quick burst of light at a certain color temperature. The flash duration is usually about 1/1000th second and the quality of light generated is very similar from flash to flash. 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. This is great but you don't need to spend $300 on a flash.
I bought an inexpensive flash from the local camera store for about $75. Why? 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. I do not take pictures with the flash mounted on the camera hot shoe; I prefer to have the flash off-camera, preferably about 2-3 feet away from both the camera and the subject. The other two source of light are not flashes but AC slave strobes.
AC slave strobes look similar to regular incandescent bulbs and as the name implies, they are plugged into your wall. The burst of light from the camera flash triggers them to go off simultaneously. The strobes produce a flash of light similar to the flash. After the strobe has discharged its light, it recharges in a few seconds and is ready to go off again. Fortunately, these strobes cost only about $25 from mail order camera shops. It is a very simple and cheap method of generating additional light.
I screw in these strobes into clamp-on sockets and clamp the strobe onto the top of the tank or onto furniture, pointing at the tank or the fish. When I am taking pictures in a 55-gal. tank, I have my camera flash to the left or the right at about the same level as the camera, aiming at the fish from the side, at about a 45 angle. This angle is very important in preventing flash reflections on to the film. It is critical to review simple laws of physics when aiming light on glass. I then have two strobes on either side of the 55-gal. tank, aiming down on the fish from above. So, every time I click the shutter release, three or four sources of light go off.
Why do I use more than one source of light? Using just one flash produces a washed out fish with a dark background and harsh shadows. It is often difficult to see where the fish ends and the dark shadows begin. In addition, using a single source of light produces an image where the subject looks flat and two-dimensional. Regardless of the expertise of the photographer or the power of the flash, the images produced using just one flash are, in my opinion, inferior to those created using two or more sources of light.
Using two sources of light, both from different directions, results in an image absent of dark shadows. Bi-directional light also gives the images a three-dimensional character; this is especially true if there is light coming into the tank from the sides and overhead.
Adding an additional source of light above the subject (as opposed to light entering the tank through the front or side) is probably the single most important change of technique I can suggest. It illuminates the background and makes the fish appear almost three-dimensional. □