Listen and look inside the tank
Does the toilet run continuously or refill every few minutes? With the tank lid removed carefully, check whether the water level is reaching the overflow tube or whether the flapper is failing to seal. Keep cleaning chemicals out of the tank unless the toilet manufacturer allows them.
Try the dye test
The EPA recommends putting a few drops of food coloring in the tank and waiting about 10 minutes without flushing. If color appears in the bowl, water is passing from the tank to the bowl. A worn flapper is a common cause. Flush afterward so the dye does not stain.
Check for an external leak too
Water at the base, behind the toilet, or around the supply connection is a separate issue. If the floor is wet or soft, turn off the toilet’s supply valve if it operates normally and arrange service; a tank-part replacement will not address damage outside the tank.
Repair based on the result
A correctly matched flapper or fill component can solve many simple tank problems. If parts are unfamiliar, the shutoff does not work, the leak returns, or the toilet moves at the base, ask a plumber to diagnose the assembly.
Technical reference: US EPA WaterSense: home maintenance and toilet leak test ↗