UPDATE *** UPDATE *** UPDATE

1. Plugged main drain with a 2" plug, complete with a wing nut. (Got it from Home Cheapo, dove the drain, and plugged the thing tight.)

***After 30 hrs, still water loss of about 1"

2. Kept the main drain plug in, bought another plug and plugged the skimmer. ***After 30 more hours, only about 1/8" water loss.

3. Bucket shows about 1/8 water loss after 30 hours.

4. I am going to pull the main drain plug now, and see if I've found the problem: The skimmer lines.

Let you know.