Can res temps get too low?

5 gallon dwc. 20 oz frozen water bottle thrown in in the morning. Do i have to worry about temps dropping too much or is it a non-issue?

too cold is certainly better than too warm. I think the plants prefer 60-70 for the res temp. i probably wouldn't it get much below 40-45F i would guess. but really if the water doesn't freeze and the plants don't die then you're doing good.