I'd go with six stacks of blocks; one at each corner, one in the middle front and one in the middle rear. The blocks would lay lengthwise, i.e. oriented the same as the tank.
Somebody will inevitably tell you that the blocks need to be stacked so that the channels in each block are oriented vertically, i.e. each stack will look like a solid smooth column with no openings visible. While that is technically the strongest configuration, these blocks have such massive strength that I always laid them the other way, i.e. with the open channels facing sidewise leaving all those little cubbies for storage or to route hoses, airlines, etc. It's somewhat weaker in terms of resisting the weight pressing down upon them, but still so far beyond what is needed that there's nothing to worry about. Strong enough is strong enough. Using six stacks of blocks is already way overkill, but doing so means that weight is being supported by concrete rather than just the wood frame.
Your framework of 2x4's will, of course, be constructed so that the wood is standing on edge rather than just laid down flat.
You can level the tank by stacking the blocks and then shimming between the stacks and the wood framework (the dumb way) or by making sure the floor itself is level before placing the stacks (the smart way). Having done it both ways in the past, I strongly recommend the smart way.
