Honestly, the only way to "repair" silicone is to tear the tank down and rebuild it. Old silicone does not stick to new silicone, and it's and exercise in futility to think that cutting out the fillet and replacing it is doing anything to "reseal" the tank. That fillet or "finger smear" along the inside of most cheap glass tanks is just an easy way for a builder to deal with the excess silicone that squishes out when the panels come together.