High CH after using metal brush for Black Algae?

[FlyAU98]

I finally realized the couple dark spots in the pool (that had been there since we bought the house last year) were black algae. It really didn't look a lot like the pictures here and never grew...so I didn't think thats what it was.

It appears to have etched itself into the finish a bit, but after over a week of brushing with a metal brush, it appears to be gone.

When brushing the sides with the metal brush, I got some cloudiness that would come up in the area I brushed. But it went away pretty quickly. Though I have some staining, I never saw anything I would call calcium scaling, the aluminum ladder looks normal...

But now by CH is up to 425...up 100 from a month ago. Confirmed over separate tests about a week apart. I believe my testing procedure was correct, as I've had consistent results over the past year until now.

Today's #s:
pH: 7.9
FC: 3.5 (I had not added today's Chlorine before the test)
CC: 0
TC: 3.5
CH: 425
TA: 70

CYA from a couple weeks ago was about 55, nothing has been added that should have changed that.
Borates are about 50
Salt was at 2300ppm back in May..this is NOT a salt water pool. I suspect this is all from liquid Chlorine or perhaps salt added by the previous owner. I haven't added any.

Water temp stays in the mid to upper 80s. Full sun all day until I get my sail-shade installed this week.

 

[chem geek]

An increase of 100 ppm CH would be around 13 pounds of calcium carbonate dissolved into the pool. However, if that occurred, your TA would have risen by the same amount (100 ppm) and the pH would be through the roof (over 9). For the CH to rise and not the TA or pH, the calcium would have to come from a salt like calcium chloride, but that seems unlikely unless some earlier calcium chloride addition somehow got stuck in the plaster surface bound to algae or something like that. So there's a mystery here.

Also, if I were to assume 13 pounds of calcium carbonate evenly distributed throughout a 15'x30'x4.5'(avg) pool, then that is ((5867 g) / (2.71 g/cm³)) / (418064 + 2*62710 + 2*125419 cm²) = 0.00273 cm or 0.0273 mm. So this isn't very much thickness of removed plaster, but again it doesn't seem like it was calcium carbonate that was removed.

 

[FlyAU98]

Thats good math

Regardless of its source...assuming it doesn't continue to creep up, do I just leave it alone?

I guess there is a possibility of it coming from fill water...with South Florida's high temps, cloudless days and no rain this summer, I've been adding water a bit more than usual. I'll check that next time I have the kit out...

 

[chem geek]

You shouldn't worry about it unless your pH and TA are very high so that the saturation index is high enough to imply scaling. However, it only increases the saturation index by 0.1 units so I wouldn't worry about it.

 

[FlyAU98]

Thanks!