If water is trickling into your overflow tube, the tank level is set too high. The most common culprit is a float that needs adjustment, but if the water level keeps creeping back up after the fill valve closes, the valve itself is worn and needs replacement by a licensed plumber.
How the overflow tube works
The overflow tube is the vertical standpipe in the center of the tank, connected to the flush valve. It’s a safety drain: if the fill valve fails to shut off, water goes down the tube into the bowl instead of onto your floor. That’s the design working correctly. What’s not correct is water running down that tube on every flush cycle, which means your toilet is silently wasting water around the clock.
You’ll often hear this as a faint hiss or trickling that doesn’t stop after a flush. Some homeowners only find it when they lift the lid and see water right at the rim of the overflow tube.
The correct water level
Tank water should sit about 1 inch below the top of the overflow tube. Water at or above that line drains continuously. Some tanks have a fill line marked on the inside wall; if yours doesn’t, that 1-inch gap from the tube top is your reference.
Note: the “CL” mark on most fill valve bodies refers to the valve’s anti-siphon positioning, not the target water level. For water level purposes, use the overflow tube top.
Why the level gets too high
Float set too high. The float signals the fill valve when to stop. If it’s set too high, the fill valve won’t cut off until the water is already spilling into the overflow tube. This is the most common cause.
Fill valve that won’t fully shut off. Fill valve seals wear out. Once the internal seal or diaphragm degrades, the valve lets water trickle past even when the float is correctly positioned. You’ll see the water level look fine right after a fill cycle, then slowly creep back up to the tube. That’s a valve seal problem, not a float problem, and adjusting the float won’t fix it.
Waterlogged float. On older ball-float designs, the float can crack and fill with water, losing buoyancy. It never rises far enough to trigger shutoff, so the tank keeps filling until the overflow tube drains it. Modern cup-float designs don’t have this issue.
Fill valve at the wrong height. Most modern fill valves are height-adjustable. A valve body sitting too low can put the effective shutoff point above the overflow tube top.
What to look at first
Lift the lid. Mark the current water level with a piece of tape on the tank wall, flush, let it refill, and see where the water stops relative to the tape and the overflow tube top. If it stops above the 1-inch line, that confirms overfill.
This observation is useful to describe to a plumber. It tells them whether they’re likely adjusting a float, replacing a seal, or dealing with something else entirely.
What a plumber checks
A licensed plumber will verify the fill valve height is correct for your specific tank, watch a full fill cycle to separate a float issue from a valve leak, and inspect the flush valve and flapper at the same time. If the overflow tube height doesn’t match the flush valve, they’ll catch that too.
Running toilets are usually a quick repair. But if the toilet also rocks on the floor, has corrosion at the base, or has gone through multiple flapper replacements without fixing the running sound, one plumber visit covers it all rather than chasing problems separately.
Call a licensed plumber
A toilet draining continuously into the overflow tube wastes more water than most people expect. The underlying cause is almost always one of the issues above, but diagnosing and confirming the fix held takes someone who can see the whole tank assembly.
Before you hire anyone, verify their license at cslb.ca.gov. California plumbers need a C-36 license for this kind of work. The lookup takes about 30 seconds and protects you from unlicensed repairs that may not be done to code.