You'd think that, but you might very well be wrong. It's a common misconception that any concrete slab will support an aquarium of any size. I have a bit of experience with tanks in that size range and larger, and the first step is to hire a structural engineer to make sure that the slab is up to the task you're asking of it.
One reason I can think of why not to use block is that it's more difficult to brace against the water pressure. For the walls you can build a 2x6 wall structure but go 12" OC instead of 16. 2 sheets of 3/4 plywood should hold the pressure. You will still need bracing across the top. Use some type of pond liner or bituthane (the sticky rubber used on roofing up north) for sealing. 4" 3000psi concrete should hold the weight.
I think with that kind of weight more than just a concrete slab will be required.. Im thinking this will be full on structural like you are building a house because its gonna weigh as much as a house.. Gonna probably need a full foundation under that slab...
