check the coldwater section there is a pinned article, but the options are limited to those until you get a heater, which seems a worthy investment then you could get a large southamerican cichlid or frontosa, these make great pondfish, dependent on size still though
maybe green (European) tench (tinca tinca) or white cloud mountain minnow (tanichthys albonubes), chinese blue bitterling (rhodeus sericeus) or the rainbow dance (notropis lutrensis)?
I believe they are some fish that don't mind colderwaters (indoor pond is not that cold really)
If I had that size of a pond, I would put a couple of ID sharks in it...and maybe an arowana...but yeah...I would really look into getting that pond heated as it will really expand your options.
Rosy reds work well in coldwater ponds, and if you have other types of fish (and a big enough pond) they serve well as feeder fish that breed themselves...