First off, great rack set-up! I personally use 1/2" MDF for any shelving/stand/rack-setups that I build. Although, MDF is a little more expensive than plywood, and plywood can work just fine. On your set-up, I would suggest at least one more support on each shelf, in order to prevent the shelves from bowing. If you cut strips of wood the width of the shelves (like the ones supporting the shelves on the sides, then tack these to the back-board directly under each shelf. A strip can also be placed in the front too, but you may not have room for this (because the tanks/containers may not be able to pull out afterwards. But one on each shelf along the back will definitely help and correct the bowing problem. Good luck.