Because every morning there is black “sediment “ on the bottom of the pool. Mostly where the side walls meet the bottom in the deep end. It brushes up easily (like dust) but comes back the next day.
Moved from here.
So, what’s the solution? I have the same issue. Was told it’s black algae. Poured in 4 gallons of liquid shock. No change overnight. I have a sand filter, built in pool (vinyl liner) with 25,000 gallons.