[GeToChKn]

You can use GE Silicone I, but you can't just put new silicone over the leak, you have to take the silicone off fully where you want to reseal, and by fully, like fully, razor blade cleaned clean. If you're never done it, buy another tank and sell yours on craiglist for a few bucks for a reptile tank and get the money back. It takes a skill to learn to seal tanks and most people who do it, end up with a wet floor, dead livestock and a not so happy partner.

 

[GeToChKn]

Water test in a garage or basement for a few weeks first, google how to do it, watch some youtube videos, etc to get an idea of the best ways to do it. Not saying it can't be done, just too many people think slapping some old silicone on top of the old will fix it, when really you have to take out at least 1 pane of glass on the leaking side, clean at least the 3 edges it comes in contact with, 4 if it's the bottom, and then reseal all those. 1 nice slow continous bead all the way along. Using masking tape to keep your silicone not too messy.