To Fishman Dave's thoughts on temperature: I've asked this question a few times & a few places, looking for the low-end occasional temp for various species (me: silver arowana). The only workable feedback we got was somebody (apologies, don't remember who) saying they had a couple of juv RTC in a balcony tank in Florida that spent winter nights ~55F through a couple of months and the fish didn't miss a beat. My expectation is that species from rivers that get a little altitude (>1500') will know these sorts of temperatures (Amazon/RioNegro, Guyana, Mekong, Eastern Congo, southern/eastern Nile), whereas stuff from lower/older islands & floodplain won't appreciate; IME clown loaches start getting timid at 25C get very quiet at 21C, for example, though I can't speak to where or how long until when health suffers.
So long as it's not particularly shallow with a dark (mud?) bottom, a pond that size isn't going to track pulses in air temp very closely unless there's a big rain. If the 50F is an occasional overnight, but back over 70F as the sun comes up (my house) then I'd not worry too much. Keep me/us posted either way.