MDF isn't very vulnerable to moisture. Ordinary particle board is vulnerable and swells badly when wet. The only disadvantage of MDF is that it isn't very strong in bending. So, if the leveler or other foot is under the bottom and not under the side piece, the weight of the tank tries to bend the bottom, to bow it upwards. But, if the leveler is attached to the side and not to the under side of the bottom it should work fine. (I don't remember how the ones I used are attached.)
Plywood is stronger than MDF, and lighter in weight, but to use it you have to cover the raw edges of the plywood, which can be a nuisance. Also, most plywood we can buy without paying a big price is warped before we ever get it. Even if it isn't I always found it nearly impossible to keep it from warping after I cut pieces to size. To coin a phrase - they don't make it like they used to.
Alright then MDF it is
. It should be ok for a 24" X 18" surface with 3/4" inch thickness, given my 48" X 18" MDF stand holding my 80 gallon is doing perfectly ok for so long. Thanks again!!