Most will say under 8 deg don't feed and under 12 deg wheat germ, Not sure if there is any real proof this is correct as wild carp don't have the choice and fishermen still catch them in low temps.
I don't drop below 12 deg now & I am trialling feeding wheat germ all year round just as a project.
Before I heated I used to feed if the fish would take it and use the rule if any food is still floating in the pond after 5 mins you have given too much, your fish will show no interest if the temp is too low and conserve energy by keeping dormant (Hate it when I visit ponds and see loads of swollen uneaten food hanging around the pond).
If you are unheated try to insulate as much as possible air pumps I would say are not really necessary at low temps as they can chill with cold air water retains oxygen saturation better at low temps, keep pumps running through filters and UVs on they will generate slight heat, Try to avoid water passing through cold air ie waterfalls showers ( would insulate showers) and if poss sheet the pond with a gap for gassing. I fully sheet night time and slide back one sheet slightly through the day.
Please don't take this as gospel this is just my way.