bz, An opening of only one ft may be part of the reason as you are not exposing a large area of the pond surface to the frigid air temps.You are not circulating a tremendous amont of water (a good thing)If you had 5 cfm going into an AirPod or Vertex XL you would have 40-50 ft of open water(not good) no matter the temps and a lot of stressed fish. 5 cfm in 1/2 acre is a lot of air inputDo you know what your back pressure reading is and are you using a vane or piston (just curious)Is your stone setup a AES ALA 4GL four square stones on a poly base?