adjust cya before or after shocking

[BigDave]

**** Can algae, over time, raise its tolerance to FC? That is to say, if you don't get all the algae when you shock (due to human error or whatever), at some point in time after repeated unsuccessful attempts to shock, will the FC need to be maintained > 3 - 5 ppm in the future to keep algae at bay?

I don't know if algae can become more cl tolerant through selection, makes some sense that you may be cultivating a cl resistant algea population. I do know that once you've raised the FC high enough for long enough to kill that population, you won't have to be concerned raising your base cl level for the resistant population as it will be gone. Any new algea that enters the pool will not have been through the selction process and will be no more cl tolerant than all the other algae.

[Kateyru]

Touching base again. I started my 2nd shock attack on 5/8 after messing up the first time, i.e., I didn't keep FC > 15 ppm long enough to kill all the algae. About the time I started 2nd attack, I ran out of reagent so used dilution testing until new reagent arrived. Using dilution testing then FC testing with K-2006 kit when replacement reagent arrived, I've tried to keep FC at or above 15. So, this is day 12. I was out of town for a couple of days this past week and am aware that FC got below 15 a couple of times. How much of a setback is that? I didn't lose any FC from last night until this morning. My plan, per forum guidance, is to stay at or above 15 for an extra day, that is, 1 full day after no chlorine loss overnight. That would be today if the 2 recent dips below 15 don't have a significant effect overall. Guidance please.
Thanks!

[Watermom]

You are right to give it the extra day as long as there is still no more than a 1ppm chlorine loss overnight from sundown to sunup and you don't have a CC reading.

[Kateyru]

FC was at 17 at 7:30 am. I added 3 qts. of bleach at that time since I knew I would be gone for a few hours. At 1:30 pm, FC was 15. I added another 3 qts. of bleach. At 4:30 pm, FC was 16. I was gone again for a few hours. I didn't add more bleach since it was 4:30 in the afternoon. Got home and checked FC again at 8:30 pm, and it was 13.5. I lost 2.5 ppm between 4:30 and 8:30 in the late afternoon/evening. Seems like a lot. I've added another 3 qts of bleach now (at 8:30 pm). That seemed the way to go rather than worrying about chlorine loss. Is this loss excessive for time of day? Sun sets about 7:30 pm and it's hot (mid 80's) but sun is off the pool by 5:30 pm or so. So, is the loss due to sunlight? Does stubborn, refuse-to-die algae exist???? I feel like I have been shocking for ages.
Thanks.

[PoolDoc]

Bleach loss during the day is likely sunlight. You can really only tell what your non-solar loss is, by comparing late PM to early AM readings.

What is your CYA level?

[Kateyru]

The last time I checked CYA was 5/8 and it was 40. I've been following Janet's recommendation to focus on FC and not worry about anything else. This morning at 6:30, FC was 16.5 and CC = 0. I had not been checking CC but did today since Watermom said to. I added 3 more qts of bleach and turned SWG on as I was not sure I could get home until late afternoon. Trying to make sure FC stays over 15.

[PoolDoc]

Do you still have visible algae?
Where? What does it look like?

Also, it's probably time to retest your CYA level.