In the evening when my pool pump turns off, my spa is draining back into the pool until it reaches the same water level. This is my first pool. It is not a new build pool. It came with the house we moved in to about a month ago. The spa is slightly higher than the pool with a water fall flow-over effect. I just started noticing it a couple days ago. I cannot remember if the pool/spa was always doing this. From my research the check valve stops the spa water from flowing back into the pool due to gravity. I have included a picture of return jet plumbing. I cannot seem to locate the check valve on my system. From talking with someone at Leslie's I was informed I either do not have a check valve in my system or the check valve is under ground. When I turn the skimmer and return valves to spa on / pool off at night (when the pump is off), the water stays in the spa. So either I do not have a check valve, or I cannot find the check valve, or it's hidden under ground. Since the pool/spa is plaster will this be a problem with cracking. I am noticing some lines on the seats in the spa. I am not sure if they were already there before or if this is new. I have heard different things about the spa draining back into the pool. One thing I heard is that's good because the pool chemistry will balance better with the spa. Another is that plaster cracks when exposed to sunlight/heat. I live in Las Vegas, so yea, we have a lot of heat. I am not sure what I should do at this point. Just close the pool valves at night? Install a check valve? Look for the check valve? Any advice would be appreciated. Thanks!