fighting mustard algae

[BigDave]

As I recall, you weren't able to open the light niche. It could be lurking in there. I believe ChemGeek advised that you'll need to keep FC above 15% of CYA to keep the yellow algae at bay if you can't get rid of it all.

[Watermom]

No need to do 'regular' shocking and then move to 'mustard algae' shock levels. Just start with the higher shock levels.

[Kateyru]

Will do! I just reviewed "mustard algae" section on TFP website. Rec is to lower pH before shocking. Do I need to do that or just shock at higher level for mustard algae? I need to maintain mustard algae shock level for 24 hours, correct?
Thanks.

[chem geek]

The pH rises when you increase the FC levels and for yellow/mustard algae you raise the FC A LOT. Lowering the pH first helps to lessen how far the pH rises and has the active chlorine level be higher as well. Ironically, having CYA in the water has the pH go even higher when you add chlorine compared to no CYA. For example, using a TA of 80, CYA of 70 ppm I get the following pH changes from raising the FC by the amounts indicated when using a hypochlorite source of chlorine.

Regular Shocking (from 5 ppm to 28 ppm FC so increasing 23 ppm FC)
pH goes from 7.5 to 8.7 (active chlorine 0.21)
pH goes from 7.2 to 8.2 (active chlorine 0.26)
pH goes from 7.1 to 7.9 (active chlorine 0.28)
pH goes from 7.0 to 7.5 (active chlorine 0.31)

Yellow/Mustard Shocking (from 5 ppm to 42 ppm FC so increasing 37 ppm FC)
pH goes from 7.5 to 9.0 (active chlorine 0.35)
pH goes from 7.2 to 8.7 (active chlorine 0.45)
pH goes from 7.1 to 8.5 (active chlorine 0.51)
pH goes from 7.0 to 8.3 (active chlorine 0.59)

The pH should drop back down at least part of the way as the FC drops, but during the shock treatment it would be better to not have the pH rise quite so much. Note that the main reason for the higher shock level is to get the level higher even in areas of poor circulation and to kill algae that has been established so has more protective layers (or is thicker). Technically, if you were able to access the algae wherever it hides and break it up, it wouldn't take so high a chlorine level.

If there were no CYA in the water, then adding 37 ppm FC with a starting pH of 7.5 would go to a pH of 8.0 instead of 9.0 shown above. Of course, with no CYA in the water one wouldn't need to shock so high (but chlorine would break down from sunlight quickly).

As for the advice about going to a regular shock level first, that's for pools that have a mixture of algae problems or where the overnight chlorine demand is high. The regular shock level gets rid of any green algae and chlorine demand without losing so much during the day from sunlight. After that stage, then shocking at the higher yellow/mustard alage level is done. In your case, you don't have other problems or chlorine demand so can start off right with the higher yellow/mustard algae shock level. This is the problem with trying to codify these sorts of things into rules -- there are too many unique situations to have a "one instruction for everyone" so posted instructions need to be seen more as guidelines while the forum experts can guide more specifically. Nevertheless, Ben's instructions are very thorough and work well in most cases; the TFP ones tend to be shorter and simpler and hence have more exceptions. Also, some things I've recommended (such as lowering the pH before shocking) don't get into such rules.

[Kateyru]

I'm a little confused about...
1) whether I should lower the pH now. Most recent test results on 7/31 before I started shocking:
FC = 8
pH = 8
TA = 48
CYA = 60 to 70

At this point, I'm only testing FC as I know that pH test won't be accurate. So, should I lower pH based on it being 8 (before shocking)?

2) what is mustard algae shock level for CYA of 60 to 70? 35 to 40 ppm per TFP chart or 42 per your message? How long do I maintain that level?
Thanks.

[chem geek]

Yes I would lower the pH now. From what we have more recently learned, the pH test is actually not inaccurate at first even at higher FC levels when there is CYA in the water. At high FC what you will find with the pH test is that it initially gives the correct reading and then over about a minute or so it turns the purple color caused by the high FC oxidizing the indicator dye. Apparently, the lower active chlorine level from the presence of CYA slows down the rate of the FC interference in the pH test. So your pH reading of 8 is likely correct and you should lower the pH quite a bit (at least to 7.2 if not 7.0) before shocking.

As for the yellow/mustard algae shock level, you don't have to be exact about this. 35-40 ppm FC is fine. It isn't like there's a magic number where it works -- higher just goes faster.

[Kateyru]

pH = 8 was before I started shocking...which I'm doing now. I'll add acid to lower pH now.
I'm such a "rule-follower" that I try very hard to do exactly the right number...glad to know it's not that exact. How long do I maintain mustard algae shock level?
Final, for now, question...previously you said that I could either keep FC at 15% of CYA, use polyquat 60 weekly or use phosphorus cleaner weekly re: mustard algae (and possibility/likelihood of not getting rid of all of it since I couldn't remove my light and clean the niche). Will any of those options actually get rid of the mustard algae or just keep it out of sight?
Thanks so much!!!

a few minutes later...
Hmmmmm...I just tested FC. This am at 7, FC = 41.5. I had to be away most of the day so turned SWG on at 50%. Just tested at 4:15 pm, and FC = 23.5. Quite a drop, considering SWG was running. So, do I start all over re: shocking? Just added acid to lower pH. How long before I add bleach?