1/4 inch plywood on 36 inch centres so that you have a story to tell.
If my freezer, (on rollers so the weight isnt that spread out )with a half beef isn't going through my regular floor, I'm sure that this won't either. Or a bath tub full of water and my fat arse. If you are really concerned, put a square of 1/2 inch plywood under it to spread the weight out but I expect you will be fine with regular construction specs.