Yep, well water is rarely oxygenated. Shoot it into the air or run it over some baffles prior to it getting into the pond. No idea what causes the oil slick. Assume that you have had the water tested for safety.
It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.
Without a sense of urgency, Nothing ever gets done.
Boy, if I say "sic em", you'd better look for something to bite. Sam Shelley Rancher and Farmer Muleshoe Texas 1892-1985 RIP