is a normal behavor bc there natural waters lacked real oxygen content bieng stagnet and 90+ degrees hotter water carrys ALOT less oxygen in the water i assume around 10ppm is there normal state and 20ppm bieng what is probly in your tank. thats still .05% oxygen compare to what the 20% that air is maybe im just rambling but i really notice when my water goes down to below 70 degrees the Cks dont even move there mouths to breathe