It shouldn't be too hard to find -- assuming your pool is 20K gallons, with an average depth of 4.5', that's still a leak of over 3 gallons per minute, if you ran the pump continuously.

1. If there's NO leak with the pump off, then there's a good chance it's the backwash line, since an underground leak would continue, slowly, even with the pump off. Be sure to check the backwash discharge.

2. As noted, if there's a SLOW leak, with the pump off, you may have a leak on the pressure side of your piping. (A leak on the suction side, would either slow down, or let air in, when the pump was on.) Even though you may have lost 10,000 gallons, water won't appear at the surface if you have well-drained sandy soil.

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