Why a water tank's level drops when nobody is using water
A storage tank whose level falls while nobody is drawing water is losing it downstream, and the usual culprit is a toilet whose flapper no longer seats, which runs water from the tank to the drain all night. The chart of the level gives it away: a steady fall through the quiet hours, under the line the tank held the week before, and a monitor that keeps the chart shows it the next morning.
Trivertix keeps that chart with a solar-powered ultrasonic sensor in the tank's lid and a radio link to a receiver in the house, which shows the level with no subscription. This page is about the morning it showed one of us eight inches.
By Luke Patterson, co-founder of Trivertix. Last checked October 15, 2026.
Eight inches under the line
“Yesterday I looked at the receiver and noticed we were 8 inches below baseline, and told my wife that we’ve got to have a toilet running. And it was.”
“It has been a very dry summer. Neighbors are buying bulk water, and my well is replenishing very slowly.”
“If I had not detected the toilet running with the sensor, I would have needed to order bulk water at $400 or more. The minimum trip is $250 for 5,000 gallons.”
What eight inches means depends on the tank's width. The sums below are for an upright round tank 8 feet across, which holds about 31.3 gallons for each inch of depth at 231 cubic inches to the gallon, and on Austin Water's figure for a running toilet.
- 8 inches on this tank
- About 251 gallons
- A running toilet
- About 200 gallons a day, or about 6.4 inches a day off this tank
- So eight inches is
- About 1.3 days of it: one night and the morning after
- 7 days unnoticed
- About 1,400 gallons, or 45 inches of this tank
Roberto caught his at a day and a quarter because he looks at the receiver each morning and knew where the tank had sat every morning that month. A level alone says nothing about where it was yesterday. The chart in the app is the memory that does.
Source, read October 15, 2026: Austin Water: Water Leaks.
Source, read October 7, 2026: NIST Handbook 44 (2026), Appendix C (PDF).
What the leak would have cost
A running toilet on city water shows up on the bill. On a tank, it shows up as a truck in the drive.
The water
The leak does not stop at 251 gallons. It runs until someone finds it, and on a well that refills slowly it takes the refill with it, a day at a time.
The load
A hauler sells a load, not the gallons lost. Roberto's hauler quoted a minimum trip of $250 for 5,000 gallons, and the order he would have placed was $400 or more.
The wait
In a dry summer the neighbors are ordering too. A truck comes when it can, and the tank has to last until it does.
A directory of Texas haulers puts a load at $150 to $500, with a minimum fee of $150 to $300 at most haulers, on trucks that carry 2,000 to 6,000 gallons, and says drinking water costs more than water for a pool or a job site. The haulers' own sites, read the same day, give the size of their trucks and no price: each quotes by the address, the gallons and the day.
What a Trivertix monitor costs is announced at launch. What a load costs is above, so the sum can be done on launch day.
Source, read October 15, 2026: HaulAgua: Bulk Water Delivery in Texas.
A dry summer, and planning what you draw
The Texas Water Development Board's weekly for October 12, 2026 put 82 percent of the state in drought as of October 6, 2026, against 26 percent a year before, and said 2026 was on pace to pass 2024 as the hottest year in records that go back to 1895. In Hays County, west of Austin, the groundwater district told KUT in March that every one of its monitored wells was below its historic level and that pumps were being lowered almost every day for months. Lowering a pump costs about $1,500, replacing one about $7,500, and a deeper well upwards of $75,000.
Sources, read October 15, 2026: Texas Water Development Board: Water Weekly for the week of October 12, 2026 (PDF), KUT: Hays County wells run drier as aquifers fall to their lowest levels in more than 20 years.
On the tank, a slow well shows as a slow refill. After each draw the level climbs back more slowly than it did in spring, and reaches full later, or not at all before the next draw. The chart is a reading of the tank and not of the well: it cannot say what is left in the ground. But a household that can see the refill can plan around it: run the washer after the tank has come back, keep the garden off a tank that is still climbing, and treat a fall the refill does not cover as the leak it probably is.
Those are the two things Roberto asked for after his summer: a sudden drop flagged against the tank's baseline, and the refill rate set against it, so that what you draw tomorrow is a decision and not a surprise. Both are planned for the app, below.
What the app does today, and what this story adds
- Today
- One high and one low alert line. Cross the low line and your phone tells you, wherever you are.
- Planned: a sudden drop
- An alert when the level falls faster than the tank's usual pattern, before it reaches the low line.
- Planned: the refill
- The rate the tank refills at, set against its own baseline, so a household can plan what it draws.
- Planned: a guess at the leak
- A guess at what is leaking, from the shape and the hour of the fall. A toilet, a softener on a timer and a split line each make a different shape.
The three planned lines are not in the app yet. They are here because Roberto's morning is the case for them, and a page that promised them as done would be wrong.
Roberto caught his toilet on the receiver, which is a display for the tank level and four status lights. It plugs into the wall and joins your home Wi-Fi. It shows the level with no account and no plan. A monthly plan adds the app, on iPhone, Android and web, with the chart of the level over time and the alerts. The sensor on the tank is ultrasonic, mounted at the top of the tank and reading down to the water, reporting about every 15 minutes by LoRa radio, with no cell signal needed at the tank.
Where it fits a house on a well, and where it does not, is on the guide to a well-fed storage tank.
Finding the leak once the chart shows one
The monitor says that water is leaving and when it started. The rest is a walk through the house with the chart in your hand.
Shut a valve and watch
Close the valve into the house and leave the chart alone for an hour. A fall that stops is in the house. One that goes on is between the tank and the valve, or in the tank itself.
Listen, then dye
Lift each toilet lid and listen for water. Then put several drops of food coloring in the tank: Austin Water says that if color appears in the bowl within 30 minutes without flushing, you have a leak, and the usual cause is a worn flapper.
Read the hour
A softener that regenerates at 2 a.m., an irrigation valve stuck open and a trough float that will not shut each draw when nobody is watching. The hour the fall begins narrows it to one of them.
Read the shape
A fall that stops at the same level every time is a hole in the tank at that height, and the level where the line goes flat says where to look. The shapes a leak makes are on the guide to a stored tank.
Source, read October 15, 2026: Austin Water: Water Leaks.
Questions about a tank that drops overnight
Why is my water storage tank losing water overnight?
Because something downstream is running while nobody is using water. Most often it is a toilet whose flapper no longer seats, which runs water from the tank to the drain all night. A weeping fitting, a split line to the house, a softener regenerating or an irrigation valve stuck open do the same. Shut the valve into the house and watch the level: a fall that stops is in the house, and one that goes on is between the tank and the valve.
How much water does a running toilet waste?
About 200 gallons a day, by Austin Water's figure, which is about 6.4 inches a day off a storage tank 8 feet across. A week of it is about 1,400 gallons, gone to the drain. On a well that refills slowly in a dry summer, that is a week of refill gone with it.
How much does a load of hauled water cost in Texas?
A directory of Texas haulers puts a load at $150 to $500, with a minimum fee of $150 to $300 at most haulers, and drinking water costs more than water for a pool or a job site. Most trucks carry 2,000 to 6,000 gallons. One of our founders was quoted a minimum trip of $250 for 5,000 gallons in October 2026. Few haulers publish a price, so call with your address, the gallons you need and how far the hose has to run.
Can a tank monitor tell me what is leaking?
Not by itself. It reads the level in the tank, not the pipes, and what it gives you is the shape of the fall and the hour it began. A steady fall through the night at a toilet's rate points at a fixture. A fall that stops at the same level each time is a hole in the tank at that height. A fall that begins at the same hour every night is something on a timer. A guess at the fixture from that pattern is planned for the Trivertix app, and is not in it yet.
How do I check whether a toilet is running?
Lift the tank lid and listen for water, then do the dye test. Put several drops of food coloring in the tank and wait: Austin Water says that if color appears in the bowl within 30 minutes without flushing, the toilet has a leak. The usual cause is a worn flapper, a cheap part that takes minutes to replace.
Read next
A monitor for a storage tank fed by a well
What an ordinary day looks like on the chart, a day of household use in inches, and where the monitor does not fit.
Keeping track of an emergency water storage tank
The four shapes a leak makes on the chart, and a slow leak worked in inches.
How to read a water level gauge
And how to turn inches into gallons for a tank of any width.
Be first in line.
The waitlist hears the price and the launch date before anyone else.