The related goldfish and koi have a somewhat similar way of handling low oxygen levels during winter in our ponds. They survive also for months in our ponds when the surface is totally covered by ice. Their metabolism goes low, slow breathing, no eating, no movement. When I still had ponds, it went exactly like this. The whole filtration was switched off during this cold period. I've never used an aeration during winter to keep an open space at the surface.
They usually obtain some oxygen from dry stems of marsh and aquatic plants that are stuck in the ice and do not freeze completely.
Not in my case. For the plants that I had were in another part of the backyard as being a filter. The surface of my ponds were always totally frozen during winter.
