I know ideally, they're water should be 75-78 but I want to move my turtles outside and it is starting to get cold. Where I live it doesn't get too snowy except for maybe a few times. Right now this week, the lowest it gets at night is 45 and 64 during the day. I wouldn't mind bring them inside at night or on cold days, it's just that the indoor tank is a lot smaller so I'd like to keep them outside as much as possible. I have two small heaters I can try to add to they're huge outdoor 'pond' but I don't think it'll heat all of it and I can't afford a bigger heater right now. I'll have a thermometer to track the water temp though.
So my question is: what is the lowest tempature res turtles can handle before they either get sick or hibernate? Would adding substrate help them keep warm? If so, what's the best and least dirty option? Should I bring them in at night? At what temperature of the water and/or the outside should I bring them in? Any help is appreciated, thanks!