Finding THE pool leak is hard, even when you are at the pool deck. Trying to do it via forum posts is nearly impossible. It's easy to find *a* leak; it's hard to know if *a* leak is *the* leak. It's quite common to fix several leaks and discover -- after fixing them -- that none of them are *the* leak! I'm in the middle of this right now, on a 200,000 commercial pool.
But, your paragraph I quoted above really, really concerns me.
Fiberglass pools are VERY light, relatively speaking. If the ground water level gets even a few inches above the level of the water in the pool, the pool can literally float out of the ground. Here's an extreme case, involving a concrete pool, posted on my Facebook page. In this case it's easy to see. But if your pool lifted 2 - 3", and then settled back, you might not see it. But you still might have a bunch of cracked pipes.
Putting lightweight pools (FG or liner) in wet sand is something of a risky business.
A hydrostatic valve is installed to prevent the sort of pool pop shown on the Facebook page. But it's function depends on being installed in a HEAVY pool, like a concrete pool. Because it takes so little hydrostatic pressure to lift an FG pool, a hydrostatic valve is unlikely to open and relieve pressure. Hydrostatic valves do sometimes open, and then 'stick'. To check someone has to SCUBA down, and remove the main drain cover, and check. But again I think this is unlikely on an FG pool.
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