First thing that jumped out at me was higher water temps= lower DO. I understood you to say your DO was higher in July and Aug but did not say what the water temps were then. Is it possible that your water temps were lower in July due to ice melt off? Another thing is back when I ran my diffusers 24/7 and my water temps were in the mid 80's top to bottom and my DO ran around 5.1ppm and your DO is lower even with much cooler water than mine. Maybe because there is less oxygen at higher altitudes? Anyway, best of luck this coming winter. Sorry I am not much help with your place.

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