What’s happening in the tank
Toilet tanks are simple: a fill valve lets water in, a flapper holds it back, and a float tells the fill valve when to stop. “Running” means water is escaping somewhere and the fill valve keeps topping the tank off, either over the top of the overflow tube (fill valve or float set too high) or down through a worn flapper into the bowl.
Left alone, a running toilet is one of the most expensive “minor” problems in a house: a bad flapper can leak 200+ gallons a day, which is often the entire explanation for a mysteriously high water bill.
The 60-second diagnosis
Lift the tank lid. If water is trickling into the overflow tube (the vertical pipe in the middle), the water level is set too high or the fill valve isn’t shutting off, adjust the float down, and replace the valve if it still runs. If the water level is fine but the toilet refills randomly, put a few drops of food coloring in the tank and wait 15 minutes without flushing. Color in the bowl means the flapper is letting water through.
The ten-minute fixes
All three parts are cheap, universal, and swappable without tools beyond your hands.
Untangle or resize the chain
A chain that’s too tight holds the flapper open a crack; too loose and it catches under the flapper. Leave about a half-inch of slack.
Replace the flapper
Turn off the shutoff behind the toilet, flush to empty the tank, unhook the old flapper, and clip on a new one. Match the brand or bring the old one to the store.
Replace the fill valve
Fifteen minutes with a towel and a small wrench: shut off the water, empty the tank, disconnect the supply line, swap the valve, set the water level to the marked line.
“A running toilet is a water bill problem wearing a plumbing costume, and a three-dollar flapper usually ends it.”
The bottom line
If new parts don’t stop it, or the toilet is old, weak-flushing, and needing repairs every year, the math starts favoring a new toilet over a parade of parts. But try the flapper first: it wins far more often than not.