I did notice BG dying in late autumn, which was unusual. Maybe they were ill and spread some kind of "gilly plague" under the ice!!
Kay it sounds to me like you had a fall turnover that brought up anoxic water, which means your previous compressor was not operating up to snuff, as it apparently didn't completely destratify you pond. Now with the rotary vane you should be much better off. You may have actually lost many of your bluegills in the fall and they didn't come up until later. I had that happen with some brook trout once.
And Esshup makes some good points. You need to do some maintenance of your equipment, stones, membranes. Diaphragm pumps also wear out and need parts replaced. Be sure to replace your carbon vanes once a year with your rotary. Not hard to do.
Many people put pressure gauges on their compressors so they can tell if they are losing pressure.
