For Concrete Pools: Black algae...the worst of all algae in the swimming pool to treat. This algae usually results from a prolonged absence of chlorine in the water. If the water goes green and stays green for an extended period of time...that is when it rears it's ugly
head(s)...spores of course. The algae have spores or heads which need to be brushed off with a metal brush to remove these heads (as much as manually possible) and then vacuum the swimming pool to waste.