Look, I know y’all mean well, but you didn’t actually “rescue” that betta if all you’ve done is move him into a bare, unheated, uncycled, unfiltered, sub 5 gallon tank.
It’s great that you want to help, but you need to learn the basics first or you’re gonna end up with a real dead fish, real fast.
What’s worse is when someone has one in a <5gallon and refuses to acknowledge that it’s wrong and tries to argue that bc they have friends who’s bettas are “just fine” in tiny tanks without a filter or heater that theirs is too. (more)