Black algae?

[Tallguy]

I have had a very strange problem the last couple of week. I was going away for a couple of days and put two of the Duration plastic coated capsules in the skimmer to hold the pool while away. When I came back the capsules and the skimmer basket had a black deposit on them. There was also black staining on and around the bottom drain. The next morning the black had started to spread across the liner. I brought the basket to two different pool stores neither had seen anything like it. I brought a bottle of the Black Treat brand bromide based algae cleaner which started clearing thins up right away, but it took 2 weeks of brushing and keep FC high to get it all cleared up. Finding it hard to believe a chlorine tablet could cause such a problem. I decide to experiment using another of the Duration tablets and sure enough in 2 days the black was back on the capsule and skimmer basket (which was a new one). Anyone ever heard of anything like this or have any idea how a chlorine product could cause such a problem?

Thanks,
Bill

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Oops title should read Black Algae!

 

[chem geek]

If it's algae, it usually occurs when the FC/CYA ratio gets too low. HTH Duration tabs are Cal-hypo, but perhaps they didn't dissolve quickly enough to maintain an FC that was at least 7.5% of the CYA level at all times. See the Pool School for more info including the Chlorine/CYA Chart.