clean off the old silicone from the tank and from the trim as best as possible. Over lap the silicone to areas that originally did not have silicone as much as possible. What ever oozes out around the edges, wipe it clean, what ever you missed, razor it off after it dries.
The reason you need to do as much overlap to areas that originally did not have silicone on them is that silicone does not adhere to itself. Despite your best effort, there will be residual silicone left over after your razor the old stuff off. You wast fresh , virgin surface for the new silicone to stick to.
Joel