How can I stop a running toilet?
When you can avoid using the toilet for a bit, put a little food coloring in the tank. If the water in the bowl changes color after an hour or so, the flapper isn't sealing properly and should be replaced. If the water is still clear but you see a flow of water, the fill valve isn't fully closing and water is entering via the line attached to the overflow tube. Typically, if I go through the trouble of replacing either of these, I replace the entire innards of the toilet.