Is it leaking from a seam or did it crack in the middle somewhere?

Jim is right, get some Weld-On 16. Its viscocity is like caulk. Works great for filling in spots where you need to make sure its water tight. ;)

It dries pretty quick, but I would give it till tomorrow morning so it can cure fully.

I know Regal Plastics in San Antonio sells it. Just get there before closing time!