I personally would not use just the 2x4s like a deck on top of the cinder blocks. Reason being is that if even one 2x4 is larger, warped a bit, etc the tank will not be supported evenly. I would still get a sheet of plywood(I think Home Depot, etc will cut it for you) and place that over the 2x4s. Or if you'd rather not, get a sheet of rigid foam insulation to place between the tank and 2x4s so any uneveness is leveled out.
I currently have 1/2" ply on top of my stand and its supported by 2x6s around the edge and 2 2x4s in the middle. No deflection of any kind, the stuff is strong.