Slam question

[Grinch6]

I searched the forums but didn’t see this. If I missed it, feel free to point me to an applicable thread.
PooL: 15,000 gallon Diamond Brite surface
Chemistry:
FC 3.0
CC <.2
CYA 30
TA 90
CH 260
Phosphates 150
I use a Taylor drop test. (K2006)
Clear water, new filter cartridges.
I have algae growing. Some green and some pink (I know, pink isn’t really algae). No big blooms but showing up in the corners.
I do an overnight test. The OCLT for three nights in a row show a loss of around 0.3 loss at most. I decided to slam anyway. I raise the CL (using 10.5% liquid) up to 22. Within 3 hours it drops to 9.7. I bring it back up to 22 and it’s down to 17 just a few hours past. That was during the first day. Overnight it went from 25 to 19 so it seems to be working. My question is that the OCLT was consistently minimal and if I didn’t have visible algae I would not have SLAMed. The slam results paint a very different picture. Any ideas what is going on?

 

[mknauss]

Welcome to the forum!
When you did the OCLT, did you fully brush the pool before and disturb the algae patches? Regardless, OCLT pass or not, if you have visible algae, you should follow the SLAM Process
Your CYA is 30, so your SLAM level FC is 12 ppm. No need to raise your FC so high. You lose the excess FC quickly to UV and/or damage your pool equipment.
I suggest you read ABC's of Pool Water Chemistry.

 

[Texas Splash]

Welcome! In addition, if you have a drain, remove the cover and inspect under the cover and drain area. Don't forget behind a light if there's a hollow niche behind it. Anything else in the water? Remove it or inspect carefully. Algae is sneaky for sure. Good luck.

 

[Grinch6]

I don’t know why it says I’m new here...I joined a long time ago. The member since date is way off. Anyway, thanks for the inputs. The pool has no hiding spots other than the skimmer. Rectangle 15 x 30. I was using the table by ChemGeek that said shock for mustard algae was about 19. Not having the time to check every few hours I just went a bit higher, expecting it to drop. Again, thanks.

 

[mknauss]

Mustard Algae FC levels are for 24 hours only AFTER completing the SLAM Process.

 

[Grinch6]

Thanks, I must have missed that when reading how to SLAM. I'll let it drop to 12 and keep it there. Then bring it back up for a day.

 

[Teald024]

Thanks, I must have missed that when reading how to SLAM. I'll let it drop to 12 and keep it there. Then bring it back up for a day.

Why bring it back up for a day? You likely don't have mustard algae.
Look at the criteria for ending the SLAM process. "no visible algae" is the first one. Having visible algae alone is enough to start the SLAM process. The OCLT is just a tool to help in the process, passing the OCLT alone it is not enough to make a full decision. Also by looking at where your algae developed, it is likely you have poor circulation in these areas and not enough FC got there to make the OCLT fail.
I had a similar spot in my pool. It was always the first place to collect crud and where a tiny hint of green would show if we didn't use the pool for a week. Just an area of poor circulation.