Very interesting progress. One of a couple things may be happening to allow the single diffuser to produce an open hole each day. 1. This diffuser may be receiving more air than other diffusers and more air means more bubbles, more bubbles means more water movement. 2. The air line to other diffusers has air blockage thus more air is forced to operating diffusers. If the check valve on the diffuser is leaking water will drain into the airline and when the airline is full of water an ice plug will freeze at the water line. If airline is not below ground freeze depth condensation could collect in a valley of the airline and freeze a plug. No snow early in the winter means deep frost in the ground.

If the quad diffuser operates all winter and trout survive, IMO minimally, you need only to operate it and another diffuser all winter. Low amounts of air to other diffusers may indicate they do not have enough water movement to keep open water. It also possible the other diffusers are not operating???

Do you regularly clean the windmill diffusers each fall?. If check valve on the diffuser leaks ice plugs freeze in the airline. Do you have freeze controls with alcohol on your windmills?. Do you have pressure relief valves on the windmills?

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