A friend of mine gave me a 55gallon aquarium that has been sitting outside for quite sometime. I cleaned it up and replaced one of the end pieces of glass. I let silicone dry as required. When I started filling the aquarium it had leak, which really didn't surprise me. I drained tank, let dry. I pulled all the old silicone out and resiliconed entire tank. After letting dry again, I still have a leak in the very middle of my tank?