Hey folks thanks for having me. In ground pool, 18,000 gallons. Last four days the pool has drained from full (filled it about 6 PM) to the base of the skimmer during the night. About 3 inches and the pool pump is off. Is this a vale set issue?

"Is this a vale set issue?"?? What is a vale set? If you don't see water leaking out on the ground, you obviously have a leak somewhere for that much water loss. Could be bottom drain, wall leak etc. You could never evaporate that much overnight. Need to have a leak detection company come and trace it. My son works for American Leak Detection, see if they have a location near you, they specialize in pools.