I think
@Joe Glass Cages might have been trying to give you a simple answer rather than a technical one. If you look at their tanks, they are rounded and polished. The only way you can do that is by having the front/back/sides assembled, flush trimmed, rounded, and polished (you need to have overage on the front/back panels along the vertical edges to prevent air from sucking into the joint).
Then, there might be a method where you attach the bottom to the frame, or set it in the frame, place the "wall assembly" on top of it, rig up some spacers, fill the gap with weld-on 40/42 which does a double duty by filling the gap and (very permanently) attaching the frame to the bottom and sides (FYI 3/4/16 is not a gap filler). Also this technique would elminate
I don't know that you would want to do this with 3/4/16 because the entire joint really needs to be pinned and shimmed, tight, weights on top to ensure no air creeps in or seams peel apart as the solvent evaporates, etc.
The bonuses of doing it this way is you don't really care what the bottom joint looks like because you'll never see it. You only "really" have to make the front & back roughly square, and the end panels just have to be dead on parallel on the 2 vertical edges (because the bottom edges of the wall assembly don't actually touch the bottom panel when using 40/42, and the top frame covers up any misalignment).
The downside is you have to work with 40/42, which is really strong, and you kind of have to know what you're doing.
Back to your original post though, you said you were having trouble finding a flat sheet of MDF...I don't know why I didn't answer that one right away but MDF isn't totally rigid, so if you place it on a warped table and place a tank on it, it will bend. Try taking 2 sheets of 3/4", placing one on a flat floor, wood glue, place the other on top, then weights. Let sit for a day. This will be more rigid. But if your work table sucks, you will want to use wood slats and shims under the MDF to make it flat.
You can do this with one sheet also (slats/shims). You should be able to make an MDF surface that is flat and workable without issue...