Depending on his/her weather, maybe a visible drop in water level overnight doesn't mean a problem. We have had, until today, an extended period of abnormal weather for the deep south. Temps 92-95 degrees, humidity < 30 percent !!! and nearly continuous 10-20 mph breeze. I was noticing a 1/4 inch drop nightly and began to wonder about a leak. After a couple of weeks of that, I put the solar cover back on at night and suddenly, no water loss.