Quick answer
Mark the pool level, set a bucket of pool water on the top step, mark the bucket level and run the pump off for 24 hours. If the pool falls more than the bucket, you have a leak. Do not drain the pool to investigate; keep the water level up and check the skimmer, returns, light niche and equipment pad.
What you're seeing
- The pool drops more than about 6 mm (1/4 inch) in 24 hours.
- You are adding water daily and the skimmer is still low.
- The pump loses prime, sucks air or has trouble staying flooded.
- You see a wet patch, sinking deck stone or a damp spot near the equipment.
What usually causes it
Cause 01
Normal evaporation
A GTA pool can lose 3–6 mm a day in summer, more on hot, windy days. The bucket test gives evaporation a control so you are comparing like with like.
Cause 02
Liner, seam or fitting leak
Tears at the waterline, a failed faceplate gasket or a leaking return can lose water steadily without a visible wet spot.
Cause 03
Light niche or skimmer throat
These penetrations are common leak points because the seal between the pool shell and the fitting can fail over time.
Cause 04
Underground plumbing
A pressure-side leak can show up as water loss, air in the system or a pump that will not hold prime. This needs isolation testing.
How to fix it, in order
Work through these steps once, with the pump running and the filter clean. Do not stack a second chemical dose before the first has circulated and you have retested.
- 01
Run the bucket test
Set a bucket of pool water on the top step so it sits at pool temperature. Mark both levels, turn the pump off and wait 24 hours without rain.
- 02
Check the obvious penetrations
Inspect the skimmer mouth, returns, light bezel, main drain and any steps or jets. Look for gaps, crumbling sealant or a loose faceplate.
- 03
Inspect the equipment pad
Look for drips at the pump, filter, heater and valves. A pad leak can empty the pool while looking like a plumbing problem.
- 04
Do not drain to find it
An inground shell needs water weight. Draining a vinyl pool can wrinkle the liner, and a high water table can float a shell. Keep the water level up.
- 05
Call for leak detection
A pressure test, dye test or electronic listening test can locate the leak with far less damage than exploratory digging. Fix the cause, then repair the finish.
What not to do
- Do not keep topping up and ignoring the loss; repeated fresh water changes chemistry and can hide structural damage.
- Do not drain a vinyl-liner pool to investigate a leak.
- Do not patch a leak behind the liner without fixing the source of the water loss.
Keeping it from coming back
- Mark the water level weekly during swim season so changes are obvious.
- Repair small tears and loose faceplates before they grow.
- Keep the equipment pad dry and watch for drips after service.
- Photograph the valve positions and water level before closing for winter.
When to stop and call a pro
Do not force it
Call a leak specialist if the pool loses more than a quarter inch a day, the bucket test points to a leak, the pump will not hold prime, or you see a wet patch or sinking deck. Fast action protects the shell and the surrounding landscaping.
Compare local repair helpGeneral guidance for residential pools in Ontario. Chemical doses depend on pool volume and test results; follow the product label and keep gas, electrical and major shell work with licensed trades.