Keep the tank and stay out of busy streets!!!
Don't you just hate this sort of thing?
Personally I would sand it a little if you could (take it to someones house and use their sander or a nice rubber sanding block), or just try and pick a less bowed piece of would this next time.
Are you meaning add MDF board on top of this board already on it?? Or replace the top board?
Congrats for having the wherewithal to build a stand in the first place!