It will be more than strong enough. A 30 gallon tank will weigh about 250 pounds, or about as much as a typical 6 foot overweight American. Think of how that guy can sit on what seems to be a flimsy wooden dining room chair. You could leave the second layer of 3/4 plywood off with no problem, or you could use 1/2 inch plywood. The only thing, in my opinion, to be careful of is to be sure the corners will be tightly fastened to avoid loosening from racking stresses. A screwed on metal angle inside each corner, top and bottom would do that easily.