New silicone will not adhear to old dry silicon so make sure you get all the old stuff off and do the entire tank at once. Use a razor blade to remove the old stuff and get it really clean. I can't remember what brand I used but it was not the regular aquarium brand it was an industrial strength one with a greater tensel strength. I had to order it online then I was still short 1 tube to reinstall the overflow so a had Louis from elegent order it. Elegant did carry it at one time but was out of stock at the time I needed it. Do a few google searches.
Don't skimp on the seals and get a good spread and get the seal done quickly, one or two passes with a seam tool for cauking you can get at home depot. The working time is not that long and if you keep going over the seal trying to get perfect it will bunch up and pull away from the glass. I know it looks better with a smaller bead but you won't get as much tensel strenght on the horizontal seals. You can clean it up later after it cures with a razor.