I notice there is an overhang which could cause serious stress if the plywood should warp. Is the plywood 1/2-inch or 3/4-inch? 1/2 is not likely to be strong enough for a heavy tank. Also, there should be a pad of some kind under the tank to distribute stress. And, keep in mind that if someone should stumble and bump into the tank the whole setup could go crashing to the floor. Oh well, good luck....