I don't know what your plan is , but I wouldn't want to build a plywood tank unless it was totally sheathed inside with fiberglass cloth and epoxy.
I mean a really good job , with filleted and taped corners , and the cloth weave completely filled with epoxy resin .
At that point , the plywood quality is less important - It's really just an armature for the waterproof epoxy membrane inside .
You're talking , what - 5/8" plywood ?
At 18" high , it would probably work with just a top center brace , and no additional framing , IF you built as described above .
You could make a complete one piece top plywood rim , with it cut out to leave a center brace .
It might seem funny , but you could get good info on building a plywood tank on one of the wood boat building forums - They at least have a lot of content on glass/epoxy sheathing .