I couldn't open the pictures but I think I have the same problem....plaster is less than a year old....saw 4-5 spots at end of last season...used an algecide then to no effect. Now I have over 30. Spots almost perfectly round and range in size frim a pencil eraser head to nickle size. Almost all are in shallow end, on bottom and walls. Will not come off with sanding block, vitamin C has no effect, applied full concentrate of muratic acid, via a water bottle with acid held directly to the spot. The acid etched the plaster but the spot remains, with no change in size or color. I don't think this is biological. I think it is either copper or defects in the plaster. I would think that if it was copper the acid would have done something. I also have the nature 2 system if that makes any difference.
Small Black Spots
- Thread starter poachf18
- Start date
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Copper stains are the most difficult stains to remove from a pool. And sometimes they cannot be removed at all. You can try some dry acid, sodium bisulfate, in a sock and rub the stains with that. More here, Pool School - Pool Stains
We strongly advise against using mineral systems in your pool because they are not EPA approved as a pool sanitizer and they do not sanitize the pool. They do not kill bacteria, viruses and other human disease transmitting pathogens. Proper levels of chlorine must be used to prevent person to person disease transmission, as often stated in the mineral system owners manual. And since you already have chlorine in the pool killing the algae the mineral system is just adding copper and staining your pool for no reason.
Copper stains are the most difficult stains to remove from a pool. And sometimes they cannot be removed at all. You can try some dry acid, sodium bisulfate, in a sock and rub the stains with that. More here, Pool School - Pool Stains
We strongly advise against using mineral systems in your pool because they are not EPA approved as a pool sanitizer and they do not sanitize the pool. They do not kill bacteria, viruses and other human disease transmitting pathogens. Proper levels of chlorine must be used to prevent person to person disease transmission, as often stated in the mineral system owners manual. And since you already have chlorine in the pool killing the algae the mineral system is just adding copper and staining your pool for no reason.