tjf120 said:
I 'super-chlorinated' a couple weeks ago to kill some black algae. Haven't seen any new blooms, still scrubbing a few spots where the black is in the plaster (ie, the plaster is somewhat porous, black is below the 'surface' of the plaster)
Did you follow the shock procedure in pool school, and only stopped once you passed all 3 criteria?
Shocking is a process, not a product you dump in the pool.
Also, are you sure it is black algae? Ba is slow to appear and just as slow to go away, as it is highly chlorine resistant. It grows in layers with a hard shell between each layer. Chlorine alone will not kill it, a stainless steel brushing daily, multiple times per day is necessary to break off its CL resistant shell so the CL can then get to it. Layer, by layer, by layer. You keep the pool at shock levels until it is gone.
To verify you have BA, scrape it with your finger and smear it on a piece of paper. It should leave a dark green and black smear behind. You can also take a trichlor or dichlor tablet, place it in a sock and weigh it down on top of the spot for 1hr. After one hour, remove tablet and sock, checking the black spot. If the stain lightens from the bleaching, then its BA.
Do not leave the tablet for longer than 1hr.
Ask 1st if you have a colored plaster pool, as I'm not sure if that test is OK in such a situation.