The five-minute flapper swap that stops the phantom refills and saves gallons every day.
A toilet that hisses and refills on its own is almost always a worn flapper or a misadjusted fill valve — rarely anything that needs a plumber. We shut off the supply, drained the tank, and pulled the old rubber flapper that had gone stiff and warped.
A universal replacement flapper snapped onto the flush valve in seconds, and we reset the chain with just a little slack so it seats fully. While the tank was open we also lowered the fill level to about an inch below the overflow tube, which quietly trims water use on every flush.
The part costs under $8 and the job needs no special tools. If yours keeps running after a new flapper, the fill valve is the next suspect — a full valve swap is still a sub-$20, thirty-minute fix.