I had a 40 hex once that split a seem while I was at work. My ex-wife called and said there was a major mess on the floor. I removed the sealant that I could and like DW1230 said, just applied the crap out of it, paying special attention in trying to push as much into the edge as possible.
Didn't leak any more after that and I did it with the water still in the tank just below the spill line.