The most reliable way to check for a toilet tank leak is the food coloring test: drop a few drops of food coloring into the tank, wait 15 minutes without flushing, then look at the bowl. If color appears in the bowl, water is seeping from the tank — you have a flapper leak. Most toilet leaks are silent internal leaks like this one, where water trickles continuously from the tank into the bowl without any visible dripping or pooling on the floor. They’re easy to overlook and easy to fix once identified.

The Two Types of Toilet Leaks

It’s useful to distinguish between the two main leak categories, because they have different causes and different urgency levels.

An internal leak — the flapper leak — means water is flowing from the tank into the bowl continuously. Water exits the bowl through the normal flush path, so there’s no visible water on the floor, no damage to surrounding surfaces, and no obvious sign anything is wrong. What you do get is a silent but steady waste of water. A running toilet of this type can waste hundreds of gallons per day, which in Denver adds up on your water bill quickly.

An external leak means water is escaping outside the toilet — either from the tank hardware (the fill valve, supply line connection, or tank bolt seals) or from the base of the toilet where it meets the floor. External leaks are more obvious and more damaging because water contacts flooring, the subfloor, and potentially a ceiling below. These need immediate attention.

Signs of an Internal Flapper Leak

  • The food coloring test shows color in the bowl without flushing — definitive confirmation.
  • The toilet runs intermittently — you hear it cycling on briefly every 30–60 minutes even though no one flushed. This is the fill valve refilling the tank as water slowly leaks out through the flapper.
  • A higher-than-normal water bill without an obvious explanation.
  • You can hear a faint hissing sound near the toilet even when it hasn’t been recently flushed.

Signs of an External Leak

For external leaks, look at the connections and the base. Dry off the outside of the tank and all connections with a paper towel, then watch for moisture to reappear. Check the water supply line where it connects to the tank and where it connects to the wall stop valve — these compression fittings can loosen or degrade over time. Look at the two bolts at the bottom of the tank where it meets the toilet bowl; the rubber washers on these can crack and allow slow seepage.

At the base of the toilet, look for water at floor level or any soft, discolored, or warped flooring immediately surrounding the toilet. A wax ring that has failed — or a toilet that has shifted slightly on its flange — allows sewer gases and water to escape at the base. This is a more significant repair requiring pulling the toilet, replacing the wax ring, and confirming the flange is intact and properly positioned.

How to Fix a Flapper Leak

A leaking flapper is typically a straightforward DIY fix. Turn off the water supply at the angle stop valve behind or beside the toilet, flush to empty the tank, and lift the old flapper off the overflow tube pegs. Take it to a hardware store to match the size and style (or buy a universal flapper kit), then snap the new one in place, reattach the chain with a small amount of slack — not so much that it can get caught under the flapper — and restore the water supply. Test with the food coloring method again after 15 minutes to confirm the repair.

If a new flapper doesn’t solve the problem, the next suspect is the flush valve seat — the surface the flapper seals against. If that seat is pitted, warped, or corroded (a concern in older homes with hard water scale), the flapper can never achieve a full seal regardless of how new it is. At that point, a full flush valve or toilet replacement may be the more cost-effective path.

When to Call a Plumber

Call a plumber for leaks at the base of the toilet (possible flange or subfloor damage), persistent leaks after a flapper replacement, a fill valve that won’t stop running, or any sign of water damage to the surrounding floor. What looks like a minor toilet issue can sometimes reveal a more significant plumbing problem underneath.

Done’s plumbing team handles toilet repairs and full fixture replacements across the Denver metro. Visit our kitchen and bath plumbing page to learn more, or if you have an active leak causing damage, our emergency plumbing team is available around the clock.