I'm not sure on too cold but I would guess anything over 60 would be fine. It's just a guess though and I'm sure the answer lies around here somewhere. I see now that you were posting air temps so your water temp may actually be warmer. I would start with checking it at it's lowest and highest just to see where you are at. No since in fixing problems that don't exist.
How old are the pumps? Even though the stock pumps are cheap they should still last some time. Are your controllers a long ways from the reservoir? If so this can sometimes cause the pumps to continuously start and stop causing them to burn out. What happens is the pump empties the controller and when it stops the water in the line drains back into the controller triggering the lower float switch again. The drain pump then kicks on again to empty. Rinse and repeat. This process can go on for a long time if not until the next fill cycle. There are solutions though so find out if that is your problem or not.