Ok, a couple of quick suggestions. Start with a fast lens, use a large aperture. Consider stepping the ISO up to 400, 800, whatever your camera is capable of without degrading image quality. Take pictures when the tank is as brightly lit as possible, and avoid the flash. If detritus or microbubbles are a problem, turn off the pumps. The tripod is always very helpful, and pretty much mandatory if going close in with a macro lens.
My typical tank shooting is with a Canon Rebel XT, 100mm Macro, a Bogen/Manfrotto tripod, and a remote shutter release. Sometimes I actually prefer to back up and use my 70-200L. I also had a chance to use a 24-105L recently, and that took fantastic tank pictures. Comparing photos, I find that you get much better color saturation out of quality glass.
Glass distortion can be a problem if you're not shooting perpendicular to the side of the tank, and can become more apparent the closer you get (macro in paricular). If you have a bowfront tank, it's a lot harder to avoid this problem, the solution might just be to back up. On the flip side, wide angle lenses seem much less sensitive to this type of distortion than long focal lengths.
Even if you do everything "right", sometimes the camera settings still won't get the white balance just right. I commonly tweak tank pictures in photoshop to give them a natural coloration. I tend to avoid the "auto color" fix, and use the manual color balance sliders.
