A few winters ago, we had a week of very cold weather. At night it was below 32. Durring the day,it was in the 40's.
I had the heater in a 500 gallon tank. The sides were wood with 3/4 inch styro between the liner and wooden sides. At night I covered the top of the tank with a quilt.

I didn't have the water pump on at night.

I didn't have a digital controller for the heater. I used a heavey duty timmer fro HD. If I remember... I had it 'on' for 15 mins and off for a half hour. I did check the water temp for a few nights night and adjusted the timer till I was happy.

My goal was to keep the water at least in the high 50'/low 60's.