A tank leak tends to show up as a small puddle behind the toilet or drops gathering on the floor near the wall. Unlike a failed wax ring, this water is clean, because it has not passed through the bowl yet. That makes it less of a health concern, but a steady drip can still ruin flooring and baseboards, and some of these leaks get worse suddenly.
The key is patience: dry everything and trace the water upward to its highest wet point. Water runs downhill and along the porcelain, so the drip you see on the floor may be several inches from where it starts.
Where tank leaks start
| Source | What it looks like | Typical repair |
|---|---|---|
| Supply line | Wet nut at the valve or under the tank; braided line may be bulging or rusty | Replace the supply line |
| Fill valve mounting nut | Drip directly under the tank on the left side | Snug the nut slightly or replace the valve washer |
| Tank bolts | Drops forming on the bolt heads under the tank | Replace bolts and rubber washers |
| Tank-to-bowl gasket | Water running down the back of the bowl after each flush | Separate the tank and replace the gasket |
| Cracked tank | Steady seep from one spot on the porcelain | Replace the tank or the toilet |
Tracing the leak
- Close the shutoff valve only if water is actively spraying; otherwise leave it on so the leak shows itself.
- Dry the tank, bolts, valve and supply line completely.
- Hold a dry paper towel against each connection in turn, starting at the top.
- Flush once and watch the tank bolts and the joint between tank and bowl.
- Check on a dry day, too, so tank sweat does not confuse the picture.
- Take a photo of anything wet so you can show a plumber exactly where it is.
Snug it or replace it?
Overtightening is the classic mistake with toilet tanks.
Gentle snugging
- Can stop a weeping fill valve nut or supply connection
- Use a quarter turn at most, then test
- Never crank on tank bolts; porcelain cracks
Replacing the part
- Needed when washers are hard, cracked or rusted
- Tank bolts and gaskets are usually replaced as a set
- The tank comes off the bowl, so the water must be off and drained
How a tank-to-bowl repair usually goes
Shut off and empty
The water is turned off and the tank is flushed and sponged dry.
Remove the tank
The supply line is disconnected and the tank bolts are removed. Old bolts are often rusted and may need to be cut.
Replace the gasket and bolts
The large gasket on the flush valve and the bolt washers are replaced, and the porcelain surfaces are cleaned.
Reset evenly
The tank is set back and the bolts are tightened a little at a time, side to side, until it sits firm and level.
Test
The water is turned on and the toilet is flushed several times while the joint and bolts are checked.
Questions to ask
- Which connection was leaking, and was the part worn or just loose?
- Is the tank itself sound, with no hairline cracks?
- Does the shutoff valve need replacing too?
- What is the written price for the parts and labor you recommend?
Common questions
Why is there water on the floor only some days?
That pattern often points to condensation on humid days, or a leak that only appears after flushing, such as a worn tank-to-bowl gasket.
How often should a toilet supply line be replaced?
There is no single rule. Replace it any time it shows wear, and it is often sensible to replace it whenever the toilet is serviced.
Can rusted tank bolts cause a leak?
Yes. Corroded bolts and hardened washers lose their seal. They are usually replaced together.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.