My best advice from a few test samples is:
o No flash. Turn those flashes off, they'll do nothing but put a big, shiney glare on the glass and it doesn't help your photos.
o High-light aquarium. Have as much light going into the sides and top of the aquarium as you can. The more the better, because of the next step.
o High shutter speed. Especially for active fish, you'll need a high shutterspeed in order to catch a clear, crisp picture. You can't have the higher shutter speeds without a lot of light.
Some other tricks I haven't tried with (equipment, time, etc), but heard/suspect:
o Manual focus. Make a focus point, sit and wait. Sometimes the Automatic focus will pick the wrong distance, making a somewhat blury picture.
o Zoom. Get a camera with some zoom. Some cameras won't focus on anything closer then 3', and that's a wide shot. Zooms will help get the close-in look without having to be as close
o Tripod. When you zoom, every little movement by you gets magnified. Set your camera up on a tripod to stabalize it.
o Filter, filter, filter. The more clear the water is, the better it photographs. Make sure to increase your filtration to gain the clearest water available. Light will reflect off cloudy water, and also off of particles in the water. Sometimes this makes for a neat effect, but most the time it's just a pain.
o Clean. Yes, clean. That means your glass, mostly. Make sure there's no buildup on the inside or outside. Get off any fingerprints, dust, or dried water marks on the outside. Make sure the inside is free of bubles and algae growth.
Anyone have anymore?