Your Float needs lowering
The float regulates the water level in the tank. You can modify this float to customise the flushing action of your toilet. If it’s set too high, the fill valve won’t shut off and water will leak into the toilet’s overflow tube. to stop the water running continuously lower the float below the overflow tube.
Your Flapper is leaking and needs replacing
The flapper allows water to enter the toilet tank while keeping further water from entering. The water can flow continuously if there is damage to your flapper or the chain that raises and lowers it.
The Refill Tube is too long
Your toilet may run continually if the refill tubing is positioned incorrectly or is excessively lengthy. Repositioning and shortening the refill tubing will stop the toilet from running continuously.