To take these photos, I used Tv mode on the SLR, which means I forced it to use a certain shutter speed and the computer calculates the rest (aperture, auto ISO). Specifically, I used a tripod and forced the shutter to stay open for 1.6 seconds. This meant that the computer would have used the smallest aperture so that the photo wasn't over exposed. Another effect of a small aperture is that the field of depth is wide so most of the subject is in focus.
Exact specs are: Shutter 1.6s, aperture f/16, ISO 100, manual focus
Equipment: Canon 5D mkII, lens EF 50mm f/1.2 L USM, tripod, 20mm Kinko tube to shorten focal length
Editing: shot in RAW, used Canon Digital Professional, increased brightness/contrast/saturation/sharpness, cropped, and exported to 1200 pixel (long side) size
Don't fuss the details too much, most important things were the $30 tripod, shutting off the powerhead, and using Tv mode at 1.6 seconds (lots of light from my 250w metal halide in a 20" deep tank, if your light is dimmer, use a longer shutter speed)