Link? Mo was talking about it in the Camera thread in the photo section... but that's 18 pages or something now, have fun finding it!
Basically it comes down to having a remote flash mounted on top of tank, a flash controller (Canon st-e2 or another master capable flash... or with many Nikon dSLRs it's built-in) then using a very small aperture & short shutter to help freeze any movement. Think I was using f/18 or f/22 and a 1/200s shutter speed.
Then fun part is waiting for the fish to swim underneath where the flash was mounted. Patience is needed!