They don't. My neighbors on both sides have pools and use different pool guys. They come by once a week, shock, put some pucks in, and the pool looks clear for a few days. Then chlorine levels drop and they get algae. It's fine in the winter when there's less chlorine burn off but the rest of the year the pools have weekly algae breakouts. They think it's normal. I would love to go test their pools 
but I don't know how to ask lol.
but I don't know how to ask lol.
