If the tank has a perimeter reinforcement ring built in on the bottom (wood or plastic), then that perimeter takes the load. The center does not need to be supported because the perimeter part transfers the load into the sides of the tank, which have a higher area moment of inertia than the bottom due to the geometry, and therefore can withstand a higher loading. If the tank is not bound around the bottom edge, then you need to support the entire bottom of the tank as it is a load bearing surface without that perimeter reinforcement.
Last edited by Indychus; 02-26-2013 at 04:40 PM.
Reason: changed "moment of inertia" to "area moment of inertia"