The main reason for a toilet tank not filling or filling slowly is due to the toilet fill valve. The most common reasons for the fill valve to slow down or no longer fill the tank after the flush are debris issues and the length of time a fill valve has been in use.
Why is my toilet tank running but not filling?
Adjust the Fill Height by Checking the Float
The water level in the tank is controlled by an adjustable float. A float that’s set too low produces a weak flush; if it’s set too high, water spills into the toilet overflow tube and the fill valve won’t shut off. The toilet keeps running.
