They are designed to form rigidity, while fiberglass creates strength.
Quikrete 20 lb. Anchoring Cement
What do you think about putting a thin layer of cement inside a plywood build. I'm thinking about laying the tank on it's side, pouring anchoring cement, letting it dry, turn the tank on another side, pour again, etc. Since it's anchoring cement, it should bond to itself. After that, epoxy or pond armor it.
why not?
i would seal it inside with some thin steel, and get a steelworker to put it all together. Imo thats a pretty cheap metod.