I bought this house a few months ago with what I think is black algae. My CYA was off the charts so I drained. See this thread for that http://www.troublefreepool.com/need-advice-for-draining-pool-t39714.html

Here is my current pool chemistry,

FC 25
CC 0
PH 7.6
TA 120
CH 525
CYA 65

I have been well over shock level for 5 days now (between 32 and 25), and have brushed almost every day with a SS 18" brush. FC use has been very low, I have passed OCLT several days (unless you count the black spots as not crystal clear). Water is cold, weather has been in the 30s at night.

The BA is clusters of small black patches, around dime sized or smaller. They are black and slimy and can be scraped up with a fingernail. I have a lot of it, mostly on the walls and on the slopes to the floor.

First, it is very hard to brush with the big SS brush - it jams all the time, and it is hard to brush well with it - is that typical? Anyone have any tips? On a long pole, it is very hard to brush the stuff on the bottom. The brush just seems to catch on the plaster, and the best I can do, especially on the floor, is kinda drag it back over the floor and it skips along. I can brush well on the upper parts of the wall, but the lower parts and floor are almost impossible.

Second question - even when I brush areas very well, nothing comes off - the spots are not brushing off at all. I get a bit of a pale blue cloud, which I assume is plaster or something, but the spots don't come off. I assume what I am trying to do is scrub off the slime so the chlorine can work, but it doesn't seem to be doing anything.

It might be that some of the areas I have been able scrub well are getting a bit lighter looking, maybe grey looking.

So what (if anything) am I doing wrong? I have been well above shock level, and even the areas I think I am scrubbing well don't seem to be improving much or consuming much chlorine.

When you guys say to brush it, do you mean to make a couple passes over the area with the SS brush, or do you mean to scrub the area for 5 minutes with the brush until it is gone?

No matter how much I brush a spot, it doesn't go away - is that not the goal?

Can I injure the pool by too much brushing?

Oh, if that wasn't enough questions, is my ph higher because of my high FC? Should I add acid to get it down to 7.2 to make the FC work better? Should I add borates??

Thanks!

The cold water will slow down both algae growth and the rate at which chlorine will kill the algae. If there were only a few spots, then putting a Trichlor puck on them or scraping a Trichlor puck into those spots could help to concentrate the chlorine, but with the water so cold and your CYA already high that may not work. The reaction time slows down by a factor of 2 for roughly every 13ºF drop in temperature, assuming that chlorine kill time for black algae is similar to the general chlorine loss rate vs. temperature. So 72ºF is twice as slow as 85ºF, 59ºF is 4 times as slow, 46ºF is 8 times as slow as 85ºF. So what would take 1 week at 85ºF would take 2 months at 46ºF. If you believe you have removed the waxy layer from the black algae, then just keep the pool at shock level and have patience. You could brush once a week instead of every day as the algae won't be growing quickly at this temperature.

If you're getting a "pale blue cloud" when you brush, you might have a painted pool. Does any color rub off on your hand if you rub the sides? Can you post a picture?

You could also get a Wall Whale brush - it comes with a nylon head but you can remove it and screw on your SS brush head. The holes should line up per others experience - worst case you can redrill. It make scrubbing much easier as the tail forces the head down - you basically only end up having to push forward and it creates the downward force.

When we worked on a friend's pool that had black algae we attacked it with trichlor pucks and bleach and a knife!

If your water is cold now, you may not want to jump in to scrape with a knife but that was effective. Poor Bill, he had to fill a sealed bucket with barbells to give him something to hold on to as he scraped at the spots on the walls. We did let the FC level drop before we let him get in but still it was not long before his nose and lips were irritated. The CYA level is a bit tricky to be certain of (wearing polarized sunglasses is a very bad idea when reading the test) and so exactly when it is safe to be in the pool is a bit uncertain. I do think it was marginally still in shock levels when he went in with a dive mask on. When he had enough of that we took it back up to mustard algae shock levels and held it there for 24 hours or so. We had to leave shortly after and so we had to let it drop down to get the CYA adjusted up to where it needed to be for the SWG before we left town.

The spots on the floor we hit with the trichlor pucks, letting the puck sit in place for about 10 minutes per spot, then scrubbing with the SS brush for another 10 minutes or so. We found that the smaller the brush the more pressure you could apply so use the smallest brush you can find. Three of us took turns with the brush on the long pole. It was hard work and no one could keep it up for too long.

I have not been back to see that pool but I hear that since we got the CYA levels corrected the SWG is keeping up and they are monitoring it with the TF100 test kit and the spots have not returned.

Just wanted to let people know that after months of keeping my chlorine at shock or mustard shock level and scrubbing every week or so, the black algae eventually faded away. So, if you are doing this in colder weather like I was, it takes a really long time. Just keep at it! The higher the chlorine the better, and I also went to 7.0 or lower ph pretty often and I think that also helped speed it up a bit. Thanks for the help!