Likely cause: water leaking into the bowl

Why this happens

EPA says color in the bowl after 10 minutes means you have a leak. Korky says a worn flapper or flush valve seal lets water slowly leak from the tank into the bowl, so the fill valve turns on again as the level drops. This Old House's Richard Trethewey says new cold water coming in all the time would make any tank condense. Bob Vila also says a leaky flapper can be the cause of extra sweat on the tank.

How to fix it

1. Turn off the toilet's own supply stop by hand. If it will not turn, do not force it. Shut off the main water supply instead, also without forcing it, and call a plumber.
2. Flush to empty the tank, as This Old House's steps say. This also clears the dye, which EPA says to flush out so it does not stain the tank.
3. If your toilet has a tower or canister flush valve with a seal instead of a flapper, these steps do not fit it, because Korky says those need a replacement seal designed for that specific model. Call a plumber. If it has a flapper, unhook the chain from the flush lever and take the old flapper off the overflow tube. Before you fit the new one, look at the flush valve opening where it sits. Korky says to check it for debris, mineral deposits or damage, and to clean off mineral deposits so the flapper can seal. If you see cracks, warping or other damage, Korky says the flush valve needs replacing, so stop and call a licensed plumber. Then fit the new flapper so it is seated, and reattach the chain. This Old House gives these steps. Take the old flapper to the store so the new one matches, since EPA notes width and whether the flapper has a foam float.
4. With the new flapper on, make sure the chain is not holding it open, as Korky says.
5. Turn the water back on and flush a few times. Then repeat the dye test: put a few drops of food coloring in the tank, do not flush, and check the bowl after at least 10 minutes. EPA says any color in the bowl after 10 minutes means you have a leak. Flush right after the test so the dye does not stain the tank.
6. For more detail, see the running toilet page.

When to stop. If color still shows with a new flapper, or if the supply stop will not turn, stop and call a plumber. If water leaks from the shutoff valve itself, do not force or tighten it. Shut off the main water supply by hand, without forcing it, and call a plumber. If water gets on the floor, dry it so no one slips. If it reaches an outlet, plug, cord, switch or light, including a light in the ceiling below, do not touch it and call an electrician, since Ready.gov says not to touch electrical equipment if it is wet or if you are standing in water. Korky says that if the tank water level drops to a point on the overflow tube or to the bottom of the tank while the supply is off, the flush valve itself needs replacing. Call a plumber for that.

Start this troubleshooter over

FourHome is our sister brand. Browse all FourHome replacement faucet and shower parts on the FourHome Amazon store. We earn from purchases made through this link.