Hi

Have done that with both an inground pond and an outdoor 150 gallon Aquarium Used two 300 watt standard aquarium heaters in both situations

I covered both with bubble wrap during cold spells Worked BUT there was a temp slide below 40. 37 for two hours was the worst I had to deal with.

Be sure to momitor it when the sun comes up as they can easily overheat in full sunshine!!!! gary