Fiberglass is still a great way to add structure, build thickness to a plywood tank build. Technically polyester resin is not 100% impermeable to moisture but neither is epoxy or acrylic and they're used in plywood builds frequently. But on that same point, polyester does rank lower than those two on moisture permiability.
If you're concerned about that or whether polyester is fish safe or not you can always topcoat with something else, even rubberizit like you already mentioned.
Or you could just use epoxy resin on fiberglass cloth and avoid all of these concerns and the odor and fumes of polyester altogether. Since you're building this tank in place indoors I would suggest avoiding polyester anyways.
Epoxy resin is not recommended for use on fiberglass mat but works fine on cloth. And in general I recommend not using mat in plywood tanks to begin with.
