Originally Posted by JWA
What dbosman said. Learn to live with it.
Not only will fresh silicone not stick to old silicone it will also not bond well well to the glass that you thought you had cleaned off well. Silicone is almost impossible to to completely clean off glass. There is always a microscopic filling of the pores in the surface of the glass
Not completely true, I have resealed a dozen tanks at this point and all are still up and running 2-5 years later with no leaks. If you strip the silicone make sure you use lots of clean razor blades and plenty of rubbing alcohol to clean the glass with just prior to siliconing it to remove and oils from touching the glass.