The first step in identifying a pool leak is to perform what is called "The Bucket Test"
Fill a large plastic bucket with pool water and leave it on the pool steps or securely attached to the ladder. Make sure any auto-fill devices are turned off for this process.
Try to match the same water level as in the pool if possible. You may need to add a weight to prevent the bucket from floating away.
Mark the water level inside the bucket as well as the pool water level outside of the bucket.
Run the pool as normal for 24 hours and return to check the water level inside and outside of the bucket. If the water has dropped lower on the outside of the bucket than inside of the bucket, you have confirmed there is most likely a leak in the pool.
Repeat the test with all pumps turned off for another 24 hours and return to check the next day.
If you suspect the loss to be in the pool structure or liner, the next step is to remove all fittings and install winter plugs in inlets and skimmers. Repeat the bucket test once more, If the pool water continues to drop the leak is more than likely in either the pool structure, liner, or possibly in the main drain line.
After these initial tests have been completed it is time to dispatch a technician to perform a leak detection. The more information that can be provided before the technician arrives will reduce the cost of labour required to identify the leak. If the leak is suspected to be in the plumbing system, the technician will perform a pressure test of the plumbing system. If the pressure test fails, the technician will use a listening device to localize the location of the leak, even if it is underground. From there we will discuss options for repairing the leak.