Your toilet water supply line connects to your homes water supply and to the toilet tank.
Connection at the Water Supply
A toilet water supply line needs to be connected to the homes water supply. There should be a shut off valve at this location, but not always. If there is not shut off valve, you will likely need to shut the main water supply off for the house to work on it.
Toilet water supply lines require a 3/8" compression fitting at the valve. If there is no valve, there is an adapter to 3/8" compression.
We have already mentioned the water shut off valve. If the supply line is old the valve probably is also. It might be wise to change it now. See the article 'Repairing Water Shut Off Valves' for more information on what to do.
Connection at the Toilet Tank
The other end of the toilet water supply line connects to the toilet fill valve. This valve is mounted in the toilet tank and the nipple for it protrudes out of the bottom of the tank.
There are gaskets and a large nut that keeps the toilet fill valve held in place. At times a leak from this connection can look like it is coming from the water supply line.
The other end of the toilet water supply line connects to the toilet fill valve. The toilet fill valve uses a floatation device to control the amount of water in the toilet tank. Water running constantly in a tank is a sign of a poorly adjusted or bad toilet fill valve. See the article 'Fixing Toilet Fill Valves' for information on how to adjust and replace this device.