Stubborn Algae?

[YellowYoshi]

For the past few days I've been SLAMming my pool in an effort to get rid of what I *think* is a stubborn case of algae. I don't think it's metal staining, because I rubbed Vitamin C tablets against it and let them sit on top of some of the stain for a few minutes, and I noticed no change.

FC 18
CC 0
pH 7.4? (I measured pH at 7.8 before SLAMming, then added muriatic acid followed closely by chlorine without re-testing the pH. I followed the poolcalculator recommendations for the amount of acid to add. Now the pH shows something >8.0, but my understanding is that the pH test is not reliable while SLAMming)
TA 100
CH 270
CYA ~45 (This test is highly subjective. I typically just keep going until i can't see the dot at all, no matter how much I shake the tube, or how much light is shining over it. Any tips on making sure I'm checking CYA properly?)

Anyway, I've been at shock level for a few days now. The pool is plaster with Diamond Brite. Twice a day I give it a good hard brushing with a stainless steel brush, and if i really go at it hard with a lot of elbow grease, it seems to make a very slight difference. I see clouds of *something* being kicked up when I brush that I think is algae. Day to day though I feel like I'm not making much progress as the stains aren't changing their look very much.

The "algae" is maybe a greenish-yellowish-tan color. It was recently resurrected from swamp status before I purchased the home, and I'm just trying to get it all the way to clear. Any idea if I might be missing something?

Thanks!

 

[JasonLion]

Normal algae wipes off very easily with a brush/sponge/towel, and even stubborn embedded algae comes off with steel brushes. Neither will get rid of the algae permanently, but wiping/brushing will get rid of it at least visually for a while. That makes me think it is a stain rather than algae.

Vitamin C not working rules out iron stains, but that still leaves copper, calcium, and organic staining as possibilities.

It sounds like an organic stain left behind by algae. Try resting a trichlor tablet against the stain for a few minutes. Organic stains should fade significantly right where the trichlor tablet was.

 

[pwrstrk]

Your question about the CYA test. Yes it can be tricky.

When you do the test, it should be done in full sun with your back to the sun and the view tube at waist level. When the dot disappears take your reading. You can do the test more than once with the sample. Just pour the sample back into the bottle and retest. Also if the water sample is cold bring it to room temp.

 

[YellowYoshi]

I rested a trichlor tablet against the stain for a few minutes and it did appear to help. I guess this means I'm dealing with an organic stain.

I tested my CYA again using pwrstrk's suggestion and decided that a CYA of 55 is probably more accurate, so I boosted my FC up to 22. Should I expect an organic stain to fade away once my FC is high enough, or does it require a more powerful treatment? I'm brushing with a stainless steel brush to the best of my ability, but not much of the stain comes up. By the way, that stain is all along the walls of the pool, all over the steps, and in a big spot in the shallow end. The deep end seems to be clear of it.

 

[YellowYoshi]

Here's a picture of what the stains look like:

 

[jblizzle]

The organic stain should slowly fade if you maintain elevated FC levels and brush often, the FC should not need to be all the way up at SLAM level though.