I left my filter running all day yesterday and I had the return flow directed to half spa and half return jets and it creates a light waterfall effect. I dont usually do that to often. I turned the filter off last night and the spa would have been full. This morning the spa is down 6 - 8" and I was thinking because I left it in the half way position, did the water get syphoned out of the spa and back into the pool? I can think of no reason that it could possibly go down that much over such a short period of time.