If it was me I would look into using something like a JB weld. I would read the package, clean up the area to be patched. Then stand the tank as it would normally be on a flat surface with a piece of wax paper (so you don't adhere the tank to the stand) underneath that corner and then press the putty into the repair area and smooth out the exterior so it matches the plastic. Let it dry then try to match it with a little paint. I would think that would give that area at least as much strength as the plastic would