There was an overflow of water in the FT containment area causing an empty tank to float in the containment area. The overflowing water caused the tank to get pinned against a walkway. A waste treatment tech was attempting to dislodge the tank with a steel bar used for breaking up concrete and pinched his fingers between the steel bar and the tank. When he began using the bar to un-wedge the tank, the tank suddenly came loose, rolled up his right index and middle fingers and pinned them between the tank and the bar. The upward motion of the tank amputated the tip of his index finger. His middle finger was broken.