I would say it depends on where the leak is. if it's somewhere along the side seams, remove the old bead with a razor (silicone sticks to glass, but not itself unfortunately), and give a generous bead along the edge, making sure you have no air bubbles. put a plastic glove or bag over your hand and run your finger down the edge to smooth it out. if it's on the bottom however, I would agree with taking the tank apart and starting the bead from scratch. Careful with that part though, because you want to make sure you don't shift the glass (or if you do, you put it back to where it should be). Uneven pressure can be detrimental to a tank.