Get a thick, opaque solar cover.

Yes, the 12 or 16 mil opaque bubble cover is what you want. Clear covers let more sun in during the day, but at night they lose that heat faster. Opaque covers reflect the heat back into the water instead of radiating it upward.

Thickness matters because it insulates better and doesn’t rip as easily. Go with at least 12 mil, ideally 16 mil if you have a larger pool or live somewhere with cool nights. The bubble texture traps air, which is the actual insulator.

One thing nobody tells you: a cover only works if you actually put it on. Buy a reel, too, or all this advice is wasted. Taking a wet, flopping cover off a pool by hand gets old fast. A reel makes it a fifteen-second job.

You don’t need a brand name. Just look for the mil rating and “opaque” or “dark blue.” The cheap thin clear ones are for keeping leaves out, not heat in.