Toilet Tank Not Filling but Water Is Running: What to Check

If you can hear water running but the toilet tank never fills (or never stays full), water is usually leaving the tank as fast as it enters — most often through an open or leaking flapper — or the fill valve is restricted so the tank creeps up slowly while the valve keeps hissing. Start with a quick look under the lid: is the flapper seated, is the chain holding it open, and is the float allowing the fill valve to shut off?

I am not a licensed plumber. This guide walks through the usual causes using manufacturer instructions from Fluidmaster, Korky, Kohler, and Toto, plus EPA WaterSense leak-check guidance — not invented “always” diagnoses. This guide was substantially rewritten and re-fact-checked in August 2026.

Key takeaways

Running water + empty-ish tank often means a leak path out of the tank (flapper/seal, chain, trip lever) while the fill valve keeps trying to catch up. EPA WaterSense’s dye test is the simple way to confirm a silent flapper leak.
Restricted supply is the other big branch: partially closed shutoff, kinked supply line, or debris in the fill valve. Korky’s fill-valve troubleshooting starts with “shutoff fully open” and a bucket test of the supply line before blaming the valve.
Refill-tube mistakes confuse the symptom. The small tube should send water into the overflow so the bowl refills. If it is shoved down the overflow below the waterline, Kohler and Korky both warn it can siphon tank water out — which looks a lot like “water running, tank won’t stay full.”
Set tank water about ½ inch below the top of the overflow on common aftermarket valves (Fluidmaster and Korky both use that target). Follow any water-level mark stamped inside your tank if the maker provides one (Toto often does).
Hand-tighten plastic locknuts. Fluidmaster, Toto, and Korky all warn that overtightening can crack parts or cause leaks. Call a pro for stuck shutoffs, tank-to-bowl leaks, or anything that needs the tank lifted off the bowl.

What “water running but tank not filling” usually means

After a flush, two things should happen: the flapper (or canister seal) closes so the tank can hold water, and the fill valve opens until the float rises and shuts the valve off. The small refill tube also routes some of that incoming water down the overflow tube so the bowl gets its trap seal back. Toto’s install manuals call that overflow-directed flow critical to bowl refill — it is not the main way the tank itself fills.

When homeowners say the tank is not filling “even though water is running,” they are usually describing one of these patterns:

What you notice	Likely branch	First check
Valve hisses continuously; tank stays low; bowl may keep rising or swirling	Water leaving tank into bowl faster than (or as fast as) fill rate	Flapper seating, chain slack, dye test
Valve runs a long time, tank rises slowly, eventually may shut off	Restricted fill (shutoff, supply, clogged valve)	Shutoff fully open; bucket-test supply; clean/service valve
Tank fills partly, then water trickles into overflow forever	Float/water level set too high, or valve won’t shut off	Lower float; confirm valve height vs overflow
Tank water drops with supply off; no obvious drip outside tank	Internal leak (flapper/seal)	Mark level, wait 20–30 minutes (Korky’s method)

Those are related problems, but the fixes differ. Jumping straight to “replace the fill valve” wastes a trip to the store if the flapper is held open by a tight chain.

Safety and setup before you dig in

Know where the toilet shutoff is (usually a small valve on the wall or floor behind the toilet). Turn it clockwise to close. If it will not move, do not force it with a big wrench — that is a common way to snap an old valve and flood a bathroom. Call a plumber and shut water at the main if needed.
Lift the tank lid carefully and set it on a towel somewhere it cannot get kicked. Porcelain lids crack easily.
Treat tank water as dirty for hygiene. It starts as supply water, but the tank collects mineral film and whatever has floated in there. Gloves are reasonable.
Have towels and a small bucket ready before you disconnect a supply line. Some water will spill.

Step 1: Confirm whether water is escaping the tank

This is the highest-yield first move when the valve seems to run forever and the tank never builds a normal water line.

Watch a flush cycle with the lid off

Flush once and watch the flapper or canister seal. It should lift, then drop squarely onto the flush-valve opening within a few seconds.
Look at the chain: Fluidmaster’s 400A instructions call for about ½ inch of slack (a small “J” shape) so the flapper is not held slightly open. Too little slack is a classic “tank won’t stay full” cause.
Make sure nothing (extra chain links, a cleaning tablet cage, a loose clip) is trapped under the flapper.

Dye test (EPA method)

The EPA WaterSense leak checklist is straightforward: put a few drops of food coloring in the tank, wait about 10 minutes without flushing, and check the bowl. Color in the bowl means tank water is leaking past the flapper or seal. Flush afterward so dye does not stain the porcelain.

EPA’s bathroom water-efficiency guide notes that even hard-to-hear flapper leaks can waste on the order of tens of gallons per day. That matches the homeowner experience of a fill valve that keeps “topping up” a tank that never looks full.

Supply-off drop test (Korky method)

If you want a confirmation that does not rely on dye, Korky recommends shutting the supply, marking the tank waterline, and waiting about 30 minutes. A dropping level with the supply off points to a flapper/seal leak (or a crack — rarer), not a “hungry” fill valve.

Step 2: Fix the common leak-out causes

Chain, handle, and trip lever

If the flapper never seats, adjust the chain first. You want enough lift for a full flush and a little slack when the flapper is closed. If the handle sticks or the lever is bent so it never releases the flapper, replace the handle/trip-lever assembly rather than fighting a warped plastic arm. That is a small parts job for most two-piece toilets: shut water off, unhook the chain, loosen the mounting nut inside the tank (often reverse-threaded on older handles — turn the opposite way of what you expect), swap the lever, reconnect the chain, test.

Worn or warped flapper

Rubber flappers harden, curl, or get etched by chlorine and “in-tank” cleaner tablets. If the sealing surface looks chalky, sticky, or uneven — or the dye test fails after you clean debris off the seat — replace the flapper with one sized for your flush valve (common sizes are 2-inch and 3-inch; canister toilets use a different seal). Bring the old flapper to the store or match the toilet brand/model when you can. After install, set chain slack again and re-run the dye test.

If a new flapper still leaks, inspect the flush-valve seat for mineral ridges or chips. Light residue can sometimes be wiped; a damaged seat usually means replacing the flush-valve / overflow assembly — which is a bigger job than a flapper swap.

Step 3: Check the refill tube (siphon and mis-route problems)

The refill tube is the thin hose from the fill valve to the overflow. Correct routing matters more than most people expect.

Fluidmaster: water from the refill tube must flow through the overflow pipe, and the tube should be clipped so it stays aimed into that pipe — not dumping freely into the tank if you still need bowl refill, and not shoved deep into the overflow.
Korky: do not push the refill tube down into the overflow tube; that can create a siphon. Clip it to the overflow so the end stays above the tank waterline while still directing flow into the overflow.
Kohler Assist documents the same failure mode: if the hose end sits below the tank waterline, water can siphon out of the tank through the fill valve path into the bowl. Their fix is to clip the hose properly on non-canister tanks, or reconnect it to the flush-valve port on canister models (Kohler — Tank Leak / Fill Valve Hose Position).

A siphoning refill tube can empty the tank slowly or keep the fill valve cycling — exactly the “water keeps running, tank won’t stay full” complaint — even when the flapper itself is fine.

Step 4: Restricted fill — shutoff, supply line, and fill valve

If the flapper seats, the dye test is clean, and the tank still crawls upward while the valve hisses for a long time, treat it as a flow problem.

Shutoff valve and supply line

Confirm the angle stop (shutoff) is fully open — usually counterclockwise until it stops. Partially closed valves are a frequent cause after someone “barely cracked” the water back on.
Check the flexible supply line for kinks.
If flow still seems weak, use Korky’s bucket test: shut water off, disconnect the supply from the fill-valve shank, aim the line into a bucket, and open the shutoff briefly. No flow → house-side supply or shutoff problem (plumber territory). Strong flow → look at the fill-valve inlet for debris, then service or replace the valve.

Cleaning a serviceable fill valve

Sediment lodges in fill-valve inlets and diaphragms. Symptoms include delayed fill, intermittent fill, or a weak stream from the refill tube — language Toto uses in its fill-valve service notes when describing a clogged strainer.

Many modern valves can be flushed:

Shut off the water and flush to empty most of the tank.
Follow the maker’s cap-removal steps (Fluidmaster and Korky both document twist-off caps for debris checks). Hold a cup over the opening.
Turn water on for a couple of seconds to blast debris, then shut off, reassemble, and retest.

If the valve is old, cracked, or still will not fill after a cleaning, replace it. Korky generally frames fill valves as wear items on a multi-year cycle depending on water quality; that is a useful planning note, not a hard expiration date for every house.

Replacing a fill valve (careful outline)

Exact steps vary by brand. The shared sequence across Fluidmaster, Korky, and Toto manuals looks like this:

Shut off water, flush, sponge out remaining tank water.
Disconnect the supply line; remove the locknut under the tank; lift the old valve out.
Set the new valve height so the critical water-level mark sits about ½ inch below the top of the overflow (Fluidmaster and Korky). Toto tanks often have a stamped water-line mark — use that when present.
Install the shank washer correctly, seat the valve, and hand-tighten the locknut. Toto and Fluidmaster both warn against overtightening plastic parts; Fluidmaster notes overtightening can damage the valve or coupling nut and risk flooding.
Attach the refill tube to the valve nipple and clip it to the overflow (or the model-specific port). Keep the tube end from diving below the waterline.
Reconnect the supply, open the shutoff slowly, check for leaks under the tank, and adjust the float so the final waterline sits below the overflow rim.

Do not use pipe dope on these plastic supply connections — Toto explicitly says not to.

Step 5: Float and water-level adjustment (tank fills, but not enough)

Sometimes the water “runs,” then stops, and the tank still looks half empty because the float is set low. That is a weak-flush problem more than a runaway-valve problem, but it shows up in the same search queries.

Cup or float-arm valves: adjust the screw or bend carefully per the valve design so the shutoff happens near the maker’s mark.
Float-cup valves (common Fluidmaster style): tweak the adjustment clip/rod so the water stops roughly ½ inch below the overflow top.
Korky-style adjustable valves: unlocking and sliding the valve body changes the shutoff height; their install guidance targets the water-level mark about ½ inch below the overflow top.

EPA WaterSense materials say to set the refill level below the top of the overflow so the valve can shut off cleanly. Manufacturer sheets for aftermarket valves more often target about ½ inch below the overflow. My honest read: stay below the overflow, follow the mark on your tank or the valve you installed, and treat “water pouring down the overflow after the tank is ‘full’” as a too-high float or a valve that will not close.

What about a cracked overflow tube?

A damaged overflow / flush-valve body can leak tank water into the bowl and keep the fill valve working overtime. It is a real failure mode — but replacing that assembly usually means working the large locknut under the tank (and on many toilets, dealing with the tank-to-bowl gasket). That is a step up from a flapper or fill-valve swap. If you are not comfortable supporting the tank, dealing with tank bolts, and resealing the tank-to-bowl joint, hire it out. Forcing a cracked tank or cross-threading the flush-valve nut is an expensive way to “save” a $20 part.

When to call a licensed plumber

Stop DIY and call a pro if any of these apply:

The shutoff valve will not turn, spins without effect, or leaks at the packing nut when you operate it.
Water is leaking onto the floor at the supply connection, tank bolts, or base of the toilet.
Multiple fixtures are weak at once (house pressure or main-line issue).
The tank-to-bowl gasket area is wet, or you need the flush-valve / overflow assembly replaced and you do not have the tools or comfort level.
You have a one-piece, pressure-assist, or dual-flush proprietary system and parts diagrams are unclear — guessing with random “universal” kits can leave you with a toilet that will not shut off.
You smell sewer gas, hear continuous flow you cannot isolate, or a repair attempt has made flooding risk worse.

A running toilet that will not stop is also a water-bill and overflow risk. If you cannot get the fill valve to shut off after a float adjustment, shut the angle stop and schedule a repair rather than leaving it hissing overnight.

How this guide was researched

I compared the symptom patterns homeowners describe against primary install and troubleshooting documents: Fluidmaster fill-valve instructions (water level, refill-tube routing, flapper-chain slack, debris service), Korky’s fill-valve replacement and troubleshooting pages (shutoff/supply checks, siphon warning, supply-off leak test), Kohler’s assist article on refill-hose position, Toto toilet install manuals (refill-tube bowl-refill role, hand-tighten / no pipe-dope rules, strainer service clues), and EPA WaterSense materials on dye testing and keeping the waterline below the overflow. I have not tear-down-tested every valve brand in a lab. Where makers specify a half-inch waterline target, I default to the hardware you actually install.

Sources

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