Beautiful tank. A few tips to light it better for photographing.
1. Use a LOT of light. Don't rely on just the flash (in fact, unless you have an external flash you can aim, don't use it at all). Set up external lamps with the brightest bulbs they'll handle. The more light, the shorter the exposure time. Since fish are swimming, plants waving, water rippling, etc, the shorter the exposure time, the crisper, less blurry the image.
2. Avoid having the light hit the front of the tank directly. Diffuse it by bouncing it off some white poster board or something. This will keep it from washing out and give you a warmer image. This will also cut down or glare problems.
3. Try to get most of the light hitting from the two sides instead of the front. This will give it a more three dimensional appearance. Get some cardboard panels and mirrors so you can aim the light where you want it and shutter it where you don't.
4. Only use 'true blue' or 'color corrected' light bulbs. The light from a regular 'white' light bulb is not really white light. It skews to the yellow end of the spectrum giving everything a slightly jaundiced, sickly look. This effect is exaggerated by the camera which is far less sophisticated than an eye. Color corrected bulbs filter the light (by adding blue) so the end result is pure white light. This will result in truer and more vibrant color representation.
5. If you can get enough external light into the tank for a good photo, turn off the hood light. Flourescents also mess up good color.
6. Since your shooting through gallons of water and a sheet of glass (or plastic), the colors will still de-saturate a bit. Some photoshop (or equivalent) tweaking may still be necessary but don't rely on software to fix something that shouldn't be a problem in the first place. Try to get the camera to capture the truest image possible.
Experiment with these tips and your photos'll be magazine worthy in no time.