BasilB, a fast change in water temperature is always a bad thing for fish. Water from an underground source or a public water system can have a much different temperature compared to the water in a pond. My public water tends to be in the low 60's all year. That could represent a 30 degree difference in water temperature from a pond during winter. That said, the recent outdoor temperatures probably chilled the water very quickly for you and resulted in minimal temperature change to the pond. The stock tank heater I use has a thermostat that shuts the unit off once the water temperature is above freezing. I also have my heater connected to a remote power switch so I can shut it off from inside my house. I have a telescope fixed to the hole created by the bubbles and can toggle the heater depending on what I see. (LOL, what ever it takes to stay warm.) I'm currently enjoying double digit temperatures and the air pump is sufficient by itself, but I have a feeling I may need the stock tank heater again before spring.

Sorry I can't help you, our water softener/filter is only connected to our hot water line - but try calling Sub Zero on the phone. I've always found them to be very helpful and perhaps they can give you more technical info. It might help to know what kind of water softener you have.

Drain the unit thoroughly, and throw a bag or two of ice from the store into the unit before you turn it on. Your cooler will constantly need emptying because this is an ice machine - not an ice maker. The inside NEVER gets to freezing or below, NEVER. It grows ice rather than freezes it and the bottom of the pile is ALWAYS melting and getting replaced by the stuff on top. That melt off is what drains away and needs to be dealt with.

I have no experience with it, but I would see if you can check out the Perlick. I have their fridge drawers in my breakfast room. They are very quiet and I would expect the ice machines would use the same refrigeration technology. Can't tell you how well the dump factor would be insulated.