If you notice that the pool is losing a lot more water only while the filtration system is running and the filter site is leak-free, there is a high chance the leak is under the ground.
At this point a pressure test is the best option to verify which line is leaking. A basic pool set-up contains skimmers, return lines, and main drains. We are able to narrow it down by closing each line with plugs and introducing water pressure into the line to see if it holds pressure.
Most of the time if there is a leak the pressure will drop right away, however if the leak is very tiny it could take a few minutes before the pressure slowly drops. This is why having a trained professional who has performed these tests many times is important. We are able to properly diagnose which line is leaking and then perform the necessary course of actions to fix the leak.
Sometimes we can verify if the leak as at prone locations of the equipment like at the elbow of the skimmers so we don’t have to replace the whole plumbing line. This is a budget-friendly fix, however is not the preferred method because in most cases if one location is leaking then the rest of the line will potentially leak in the near future do to wear and tear.
The correct course of action is to replace the whole plumbing line from the filter site to the wall fitting. Using Schedule 40 PVC will ensure longevity of the plumbing lines.