Should I bring FC back up to shock levels?

[rlsnights]

Been fighting black algae for a while. Started letting our FC levels drop after we had several nights with no FC loss at FC shock levels.

Finally was able to afford the test kit with CYA test and discovered our CYA level was 100, not 50 like the pool store said. Today I did another partial drain and refill to try to get our CYA level down but it only brought the CYA down to 70. I'll do another partial drain this weekend to get it down to 50 but for now I am working with a CYA of 70.

I've been doing morning and evening FC tests and I'm not sure if I should be bringing the FC back up to shock levels instead of continuing to let it drop. We have started losing FC overnight but in fairly small amounts. But since I'm losing less during the day than I am at night I am very unsure what to make of this.

9/30 - 10/1 overnight lost 1 ppm (19.5 to 18.5): overnight low temp 57
10/1 - daytime loss 0.5 ppm: daytime high temp 93, pool partly shaded most of day
10/1 - 10/2 overnight lost 2.5 ppm (18 to 15.5): overnight low temp 56
10/2 - daytime loss 0.5 ppm: daytime high temp 78

Thanks for any guidance.

Patricia

15,000 IG White Plaster Cart Filter Jet Vac sweep, new pumps and filter system this year

 

[Casey]

If you are loosing more than 1 ppm over night, you need to shock because only two things consume chlorine... sunlight and organics. So, you can scratch the sunlight because it's night-time, it leaves you with organics.

 

[TizMe]

If you have been shocking thinking your pool had a CYA of 50 and it was actually 100 then you have not been adding enough chlorine to get rid of the algae.

 

[frustratedpoolmom]

Did you make any adjustments to your testing procedure? On your other post we discussed that testing in one spot while the filter had been off for a while will yield different results. So be sure your filter is running for a while before testing and get your sample from 18 inches below the surface. An overnight FC loss of 2.5 means organics so keep shocking, Tiz is right you haven't been adding enough chlorine to reach shock if your CYA was higher than you thought.

 

[rlsnights]

I changed the way I was sampling the pool and have been doing it the way you suggested: running the pump for at least an hour then taking the sample from at least 18" deep about 5' from the skimmer in the deep end.

When I was shocking the pool I had the FC levels at 35 - 42 ppm for 5 days. The last 3 days I had no overnight FC loss. That's when I started letting the FC level drop.

Based on this morning's reading it sounds like the black algae is trying to come back and I need to shock the pool again. Last night the FC level dropped to 12.5 ppm, a loss of 3 ppm (the day before I lost 2.5 ppm).

I'm going to do the partial drain/refill this morning then raise the FC based on my new CYA level.

Patricia

15,000 IG White Plaster, Jet Vac sweep, new cart filter and pumps this year

FC 12.5
CC .05
TA 120
CH 275
CYA 70

 

[rlsnights]

The FC/CYA chart says to shock at 20 ppm if my CYA is 50. Is this high enough for black algae?

Patricia

 

[JasonLion]

Shocking to 20 should be sufficient. The key to killing black algae is frequent brushing.