I am going through this right now. By no means a pool expert, but some things I've learned.
Measure your water loss. I used distance to water line from bottom of skimmer but just use something you can easily see how much you are losing after say 12 hrs.
If you are only losing water while pump is running, then you know you have a leak somewhere in the plumbing. Inspect all places where gaskets connect items together for leaks. Splitters, filter housing, pump filter cap, hose connections, return connections, etc.
If you lose water regardless of pump activity it is probably in the Shell of the pool. That is where am right now. I am having to pay a local company to come and find my leak and fix it, but I did manage to find and repair one leak. I did this using a weed sprayer filled with water and red food coloring. Walk around the pool and feel for "squishy" spots. If water is leaking through to the sand floor it will feel soft under foot. Once you find some potential problem areas, squirt a little red dye right onto the spot. If it is a hole, current will immediately suck that dye out of the pool. It is really obvious when it happens.
I realize this is all really vague, but it very much is a detective process. If you aren't a meticulous type of person find a local pool company to come do it for you. I ended up paying 100 bucks AFTER spending countless hours and money replacing hardware, gaskets, and walking around the pool. Did fix a leak, but not out of the woods. Hope this helps somehow.
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