A toilet can lose water from its tank to the bowl without making a sound. A dye test helps show whether that happens between flushes. It is useful when the toilet seems to refill on its own or when water use has increased.
Test while the toilet is idle
If you can safely lift the tank lid, set it on a stable surface. Add a few drops of food coloring to the tank and wait about 15 minutes without flushing. Color in the bowl indicates water moved from the tank. Flush after the test to clear the dye.
Use the result to narrow the repair
A leaking flapper or flush valve seal can cause this result, and a water level high enough to spill into the overflow tube can also send water into the bowl. Tell the plumber if you saw water entering that tube or heard the fill valve cycling. Check separately for visible moisture at the supply connection or base.
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