The common pleco in my experience has done ok in an unheated pond. They do much better then smaller or frail plecos. But I also have read that their taste in fish slime coats goes up, people see problems with them on their fish. If your climate is a bit warmer, but not hot enough to be tropical ive heard of people doing oscars. Other than that i would agree and say archers. I also just read that paradise fish can do well in 50 - 90f temp. Also might wanna check out bitterling, American flagfish, shubukin, sarasa, and mosquito fish.