Thanks Ben. I really don't want to pressure wash it because it is too old, (1986). I know it needs re-plastering but want to try and get a couple more years out of it. I did what you said and it is definitely black algae.
Would it hurt the plaster to pour bleach directly on the spots? I heard that you can also rub a 3" Trichlor puck on them. I am going to try and work on them now and then start the refill. Should I be pouring bleach in at nighttime when refilling, even though the pump is not running?
Thanks again.
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