Black algae--how long does it take to eliminate?

rlaw

Member
Sep 22, 2017
8
San Diego, CA
I am having trouble thoroughly eliminating all the black spots in my pool. I've been SLAMming for almost 2 weeks.

FC: between 11 and 21
CC: 0
CYA: 45
pH: 7.2

I've brought the FC up to 21 twice so far. At the beginning of the SLAM I had very low CYA so I had FC at 11. After adding 5 lbs, CYA is up to 45. When FC is at 21 I can see some reduction in the number and size of black spots. But I'm wondering how long I need to keep this up because we'd really like to swim again :)


IMG_20190815_071804.jpg
 

thetekgeek

Bronze Supporter
Jun 8, 2018
166
Nebraska
How are you testing?

FC between 11 and 21 is a huge range. I suspect you are using strips? They are not accurate enough to perform a SLAM properly. With a range that wide for FC, any advise you get here will be pure speculation. Where did the CYA 45 number come from?
 

rlaw

Member
Sep 22, 2017
8
San Diego, CA
I'm using a test kit. What I mean is I started out with low FC/CYA and then raised both using the ratios from the pool calc. I've kept the FC at the recommended SLAM level for black algae most of the time. CYA was measured using the test kit.
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
Which test kit? Can you put the model in your signature so that everyone can see?

CYA at what you perceive as 45 is rounded up to 50.

Have you been using a steel brush on those black marks? If not, then consider it. If they kinda get better at high FC, you'll need to maintain SLAM level FC and brush them. If as you say you've been between 11 and 21, when you're under 20 you're not at SLAM level and if they are black algae, they'll not respond. Maintaining FC during SLAM is essential.

FWIW, you can swim up to SLAM level FC. So if it's 19ppm or under, go ahead and swim. Just make sure to test and bring it back to SLAM level when you're done.
 

rlaw

Member
Sep 22, 2017
8
San Diego, CA
@frogabog I'm using the Taylor K-2006C Test Kit (sig updated). I scrub with a steel brush 2-3 times daily. Just re-measured the CYA and it was 50. FC was 12 so I added 2 gallons of 10% chlorine. I had just added 2 gallons this morning. So you see why I said the range is 11 to 22. I bring it up over 20 in the morning, go to work, then come home and it's 11-13. Not sure how I can avoid that unless I bring it up to 30 in the morning. Since this morning's photo I've observed some progress in the black dots shrinking or disappearing so hopefully I'm on the right track.
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
Is there anyone around who can test and add chlorine mid day while you're at work?

If not, then after work could you test and re-add hourly/bi-hourly till bed? Just hit it really hard when you're there and able. If it's black algae, it's not easy to get rid of, but maintaining SLAM level FC can help a lot. When it falls below SLAM level, it's just getting a chance to re-grow.
 

rlaw

Member
Sep 22, 2017
8
San Diego, CA
I measured FC this morning at 22 so it was definitely above SLAM level overnight. But the spots look about the same as the photo above. How long do I need to maintain SLAM level to see some improvement? I work from home today so I will make sure FC stays above SLAM level.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,718
Laughlin, NV
Read -- Black Algae - Trouble Free Pool
"Maintain an FC level between the high end of the recommended “Normal” range and the Shock (SLAM) level according to your pool’s CYA level. For example, if your pool’s CYA is 40, the high end of the “Normal” FC range is 7 ppm and the Shock/SLAM level is 16 ppm. Maintain FC within this range until all signs of black algae are gone."
 

rlaw

Member
Sep 22, 2017
8
San Diego, CA
@mknauss I've definitely kept it within that range. Is it normal to require 2 gallons of 10% chlorine a day to keep it at that level? That's around 10 FC loss per day. I think it's working but very slowly.
 

rlaw

Member
Sep 22, 2017
8
San Diego, CA
@mknauss so at a CYA of 50 I should go no higher than 50 * 20% = 10 FC? From the PoolMath calculator the suggested shock level is 20 FC.

I took a picture this morning of the same area and cropped it to the same area for comparison. It's really hard to tell if there's been much progress. I had mustard algae before and it responded very quickly to SLAM level chlorine. Is this kind of progress all I should expect?

black_algae.jpg