Silicone does not adhere to silicone. You will need to clean and prep the surface area for a new coat of silicone. It's important to note there are two parts of silicone on tanks. The joint, which provide structural integrity to the tank, this is the bond between the panes of glass. Then there is the seal, which is the part that runs along the inside corners of the tank and provides extra leak protection; which I am assuming you are referring to. You can remove the seal and replace it without removing the joint, very carefully not to compromise the joint. If you need to redo the joints, the entire tank must be disassembled.

I use RTV for my silicone jobs. RTV 108 is clear, RTV 103 is black. RTV has a higher tensile strength, which for replacing a seal, won't offer you any extra protection.