Is the sump leaking into another compartment or is it leaking out of the sump completely? If it's leaking out of the sump completely, I'd just change out the sump (or take it out, drain it, find the leak, and resilicone it if it's a glass tank. It won't give you issues -- it's like doing a water change :D Old sump w/ old water out, new sump w/ new water in ;) I've changed out my sump in my 125g 2 or 3 times since I set it up.