I Thought the Mold Was the Problem, but Cleaning Wasn’t the Solution
The mold didn’t start on the floor or around the toilet base. It showed up under the tank, where I rarely looked. Dark patches along the underside, a damp feel that never fully dried, and a smell that kept returning even after cleaning.
At first, I treated it like a surface problem. Vinegar removed what I could see. Drying helped. But the mold kept coming back.
That’s when I realized the problem wasn’t the mold. It was the tank.
Why mold forms under the toilet tank
Every time the toilet refilled, the tank stayed cold. Because the fill valve wasn’t working properly, it kept running and refilling when it shouldn’t have. Cold water inside the tank plus warm, humid bathroom air created constant condensation underneath.
That moisture never fully dried. Mold moved in.
Why cleaning alone didn’t work
Vinegar worked better than bleach on the unglazed porcelain under the tank. It removed visible mold and mineral buildup without damaging the surface. But within weeks, the underside was damp again.
As long as the tank kept sweating, cleaning only handled the surface — not the cause.
The fix that actually stopped the mold
Replacing the toilet fill valve changed everything.
Once the new valve was installed and set to the correct water level, the tank stopped running continuously. The water stabilized. The tank stopped staying cold between flushes.
Most importantly, condensation stopped forming underneath.
That’s when the mold stopped coming back.
What I used
- New toilet fill valve
- Bucket
- Crescent wrench
- Scissors
This is a beginner-level repair. It looks more complicated than it is.
The steps that mattered most
- Turning off the water and fully draining the tank
- Removing the old fill valve completely, including the washer
- Setting the new valve to the correct fill line before tightening
- Making sure the refill tube was positioned correctly
- Hand-tightening the mounting nut to avoid damaging the tank
- After reinstalling, I flushed the toilet several times and checked for leaks. Then I waited.
After replacing the fill valve, the moisture under the tank stopped completely. The porcelain stayed dry, the mold didn’t return, and nothing else in the bathroom changed — not the cleaner, not the routine, just the way the tank handled water.
This fix makes sense if you notice mold under the tank and your toilet keeps trying to refill, runs longer than it should, or leaves the tank cold and damp between flushes. In those cases, fixing the fill valve should come before replacing the toilet.
I still clean under the tank occasionally, but I’m no longer dealing with recurring mold because the conditions that caused it are gone. Cleaning removes mold, but fixing the fill valve prevents it from coming back.