Get a solar cover for overnight heat loss.

Yes, a solar cover (the bubble wrap kind) is the best for preventing heat loss overnight. It works by trapping heat in the water and, more importantly, cutting down evaporation—which is actually the biggest source of heat loss in a pool at night.

A solid thermal cover can also help, but it’s heavier, more expensive, and doesn’t let sunlight heat the water during the day. The bubble cover is cheaper, easier to handle, and does double duty: heats during the day, holds heat at night. Thicker bubbles (12 mil or so) work better than thin ones.

Just make sure you put it on before the sun goes down. If you wait until after dark, you’ve already lost a lot of that daytime heat.