A hairline crack in the reservoir tank of your toilet is a major concern. As water leaks through the crack it will widen and your water bill will rise. Once the crack widens, it is necessary to replace the tank. If you catch the problem before it gets out of control, it is possible to seal the hairline crack in your reservoir tank with an epoxy that will stop the leak and last for many years.
Turn off the water supply on the line behind the toilet. Flush the toilet to empty the reservoir. Place the bucket under the supply line connection on the bottom of the reservoir tank. Grasp the connection on the supply line with your adjustable pliers. Twist the connection counter-clockwise to remove it from the reservoir tank and allow the remaining water inside to drain into the bucket.
Wipe the inside and the outside of the reservoir tank with clean rags. Mix the epoxy as directed by the manufacturer. Apply the epoxy into the crack on the inside and the outside of the tank. Let the epoxy dry for 12 hours. Sand away the excess epoxy on the outside of the reservoir tank with steel wool.
Wrap the male end of the supply connection on the reservoir tank with Teflon tape. Twist the supply line onto the connection on the tank. Snug the connection with your pliers. Open the water valve on the supply line. Remove the bucket and check under the reservoir tank after an hour to be sure the crack is sealed.
Rubber gloves and safety glasses are required when working with epoxy.