Here's my take on it. Sunlight comeing through the ice helps the plants that will grow under the ice in cold water produce O2 during the day. Same for phytoplankton. Then once night falls, they both need to use O2 to survive. O2 is lowest right before sunrise.

12 ac pond would be hard to aerate for the winter, but if you can, I would try. The diffusers should be put at 1/4 to 1/3 the deepest part in the pond, i.e. if the pond is 30' deep, the diffusers should be between 8-10 ft. below the surface. Leave them run 24/7. The more that you can put in, the better, but I have one in my pond that averages 3/4 ac during the winter. I haven't had a winter fishkill (yet).

It could be a number of things causing the fish to die. Lack of O2, build up of Hydrogen Sulfide in the water, etc. Keeping open areas in the ice where the water can contact the air will help mitigate that.

Using alum to remove nutrients, will in turn reduce the amount of phytoplankton and plants that use O2 during periods of no or low sunlight. On the flip side, there will be less to produce O2 during the day too. Removing fish, which consume O2 will also help reduce the BOD in the pond if there is a large fish biomass.

Yes, not having all those cattails in the pond contribute to the nutrients in the pond.

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