I have an inground pool with built in spa at one end...the spa is about two feet higher than the pool surface. When the filter pump is on the water goes into the spa, over the edge, and into the pool then back to the filter pump. I run the pump 6 hours a day. By the next day the water level in the spa has dropped about 18 inches. When the filter pump starts up again for the day it takes about 15 minutes before the water level rises in the spa enough to spill the water into the pool. I have my re-fill float turned off. The water level in the pool stays the same after the filter pump is turned off. The pool/spa and equipment is about 5 years old. What causes the water level in the spa to drop after the filter pump is turned off? Is this a check valve problem and if I don't do anything to fix it will it cause any damage?