Are these spots stuck to the walls of the pool? If yes, it is very likely to is black algae and must be brushed off the sides with a wire brush because black algae has a "hard exoskeleton" that must be broken.. then an algaecide that covers black algae must be used . Pinch a Penny sells a good one. You will have to repeat the process in about a week.. hope this helped
This is supposedly an algae and is harder to kill than regular algae. The technicians at the store can help you with what chemical you need to kill it. As Stephanie says, you will need to brush.
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