Refrigerator compressor for pond aeration?
I used a old refrigerator compressor as a small shop compressor decades back with an used R12 tank. it works and is fine for shop air however I probably not do this with your fish. (Today refrigerant tanks have check valves to prevent this use and it’s not the safest method to use old propane tanks because of the thin metal used and no internal coatings in most cases).
Why not? The compressor will be lubricated with mineral oil or Poe/pag oil on newer compressors and the compressor is designed to pass oil, if using any type of compressor I would suggest oil-less or at a minimum have a trap to catch the oil.
It will only take a few drops to coat the water and could be fatal to your fish, I have a small solar water pump on my pond. It has frozen solid 3 or 4 times but has continued to work when thawed I got it from harbor freight for under 30$ and it has worked well.