In the morning you will not remember which one ran. You’re still half-caffeinated, still processing the 2 a.m. wake-up from the faint, constant running sound that snaked through the floor vents to your bedroom, so scribbling this note now saves you the hassle of checking every toilet in the house at 7 a.m. You already propped the bathroom door open three inches so you don’t walk right past it when you’re half-asleep heading for the coffee pot.

Marked Second-Floor Guest Toilet Tank

You ran a horizontal pencil line 1/8 of an inch above the current water line on the inside of the tank, right above the flapper, so you can tell immediately if the water is creeping higher when you check back. This is the only toilet in the house with a pale blue ceramic tank lid instead of plain white, so that’s your first quick visual cue before you even touch the lid. You also tucked a scrap of neon green post-it under the front edge of the seat, the one you pulled off your grocery list earlier, so even if you forget the lid color, you’ll spot the neon before you lift the seat. The pencil mark sits level with the tank lip, so you don’t have to fumble for a tape measure to confirm if water has risen above the fill line. You tested jiggling the handle twice already, and while that stops the leak temporarily, it starts back up again after 10 to 15 minutes, so you know a quick fix won’t hold long term.

Locate Its Unique Surface Imperfections

You don’t have to memorize serial numbers or weird plumbing parts to tell this tank apart from the others in the house, because it has a handful of distinct marks no other toilet has. This tank has a faint, hairline crack on the left exterior side, just below the handle, that happened when you dropped a bottle of lavender-scented toilet bowl cleaner on it last winter. The flush handle is slightly wobbly, too, because you tightened it too hard six months ago and stripped a tiny section of the threading on the mounting nut, so it swivels half an inch side to side even when it’s fastened as tight as you can get it. The inside of the tank also has a faint orange stain around the overflow tube, from the hard well water you’ve been meaning to install a whole-house softener for, that no other toilet in the house has because this one gets used far less often, so the mineral deposits build up faster without regular flushing to clear them out. You don’t have to touch the stain to confirm it’s there; it’s visible even when you lift the lid just a couple of inches, so you don’t risk dropping the heavy ceramic lid on the tile floor.

Cross-Reference Its Leak Timing

You tracked three separate instances of the leak waking you up overnight to rule out other toilets in the house, and logged them in the table below to reference later when you troubleshoot:

You don’t have to stay up all night tracking more instances; the three you have already confirm this is the tank with the issue, not the master bathroom toilet or the two downstairs by the laundry room. Illustrative example: If the leak runs consistently for 18 minutes every hour, you will waste roughly 42 gallons of water per day, enough to fill three standard 13-gallon kitchen trash cans completely. Even if the leak only runs a handful of times a day, the small volume adds up over the course of a month, so confirming the source early saves you from unexpected utility costs later.

Jot Its Exact Flush Runtime

You flushed this tank five minutes ago to test its baseline fill time, so you have a point of comparison for when you check it tomorrow. It clocked in at 57 seconds from the end of the flush to the point the fill valve clicks off completely, a full 12 seconds slower than the master bathroom toilet that got a new fill valve last spring. If you flush it tomorrow and it runs longer than that 57-second baseline, you know the flapper or fill valve is degraded enough to need replacing, rather than the leak being a one-off from a stuck handle. You don’t have to shut off the water supply to test this, just stand by the tank and count quietly once you push the handle, so you don’t risk messing with a supply line that might be stuck or corroded from years of hard water exposure. You also noted that the flapper feels slightly soft and pliable to the touch, rather than rigid and brittle, so it’s likely not completely rotted through yet, just misaligned or worn down around the edges where it seats against the drain.

Return Tomorrow To Listen For Cycling

You’ll head straight for this bathroom as soon as you pour your first mug of coffee, no detours to check the mail or feed the cat first, so you can listen for two full minutes to see if the tank starts running on its own without anyone flushing it. You’ll lift the lid first to check if the water line has risen above the pencil mark you drew earlier, which will confirm the leak is persistent enough to need a replacement part rather than a quick adjustment. You’ll test the flush one time, count the fill time out loud, and if it runs past the one minute mark without clicking off, you’ll add a universal flapper and standard fill valve to your hardware store list for the weekend. You can also press down on the flapper manually when you check to see if that stops the flow immediately, which will narrow down the issue to the flapper rather than the fill valve or overflow tube. You don’t have to fix it right away, just confirm the leak is coming from this tank so you don’t waste time troubleshooting the other three toilets in the house when you have a free afternoon to tackle the repair. You can also bring your phone with you to record the sound of the running water if it starts, so you can show it to the hardware store associate if you need help picking the right replacement part for this specific tank model.