First remember that generally, epoxies that can be purchased at local hardware stores may indeed be toxic to fish. This is because these types of epoxies have a very specific purpose intended for them and that purpose does not revolve around housing aquatic life.

A popular aquarium-safe cyanoacrylate adhesive is the Loctite Super Glue Gel Control. It forms a strong bond quickly and securely attaches small glass pieces or seals minor leaks. Apply a small amount of the gel to the surfaces, hold them together firmly, and allow the glue to cure.

For the best aquarium safe silicone, turn to Loctite Clear Silicone Waterproof Sealant. This formula can be used in saltwater as well as freshwater aquariums. It's both flexible and gap filling, making it the perfect aquarium safe sealant for any tank with a capacity of up to 30 gallons.