The way I fixed my 29 gallon is I used a razor blade and removed the old sealant. I got a tube of silicone aquarium sealant and just followed the directions on the tube. Basically, it said to clean the surface with isopropyl alcohol, dry it, apply the sealant and press it into the corner. It takes 48 hours to dry so you can fill it with water to test the seal. Make sure you don't get the sealant on your skin because it is extremely irritating and very hard to get off.