A leak in a fish tank can be a messy — and for the aquatic residents, life-threatening — issue. For a small or inexpensive tank, it is usually a good idea to simply replace the whole tank. However, if you have a large, expensive or specialized tank, replacement can be difficult. In that case, move the fish into a safe, temporary residence and repair the leak using silicone sealant. Always make sure that the sealant is marked nontoxic or pet-safe before using it on the inside or outside of a fish tank.
Locate the area of the tank, usually on a seam between glass panels, which is leaking. Circle it with the grease pencil.
Remove any old silicone or sealant along the leaking seam using the razor blade. Dispose of old sealant pieces.
Allow the silicone to cure to 24 hours before refilling the tank and replacing your fish.