FWIW: my 20L frag tank cracked from the bulkhead in the corner to the bottom. It was leaking slowly, but constant. I was able to completely seal the leak from the outside with JB waterweld. It actually bubbled up like a bubblegum bubble, but it absolutely cured and would have been basically permanent but I couldn't seal it under the bulkhead. If you are leaking around the bottom you could probably just lay a thin piece all the way around and completely seal it. It won't look pretty, but the stuff works.