It doesn't really fit my situation. I live in Northern California and this was going on even in the heat wave when the water temps were in the 90's. My water temps are now "cool" from the high 70's in the morning to the low 80's after the solar has worked for a few hours.

The only thing is that our air temperature does regularly drop below 70 at night (although it didn't during the heat wave). Maybe the spa gets cooler at night since it is such a smaller body of water. Maybe I will call the Tropical Fish people and check, but I thought this residue is a white film that floats.