Quote Originally Posted by chem geek View Post
Yellow/mustard algae is more resistant to chlorine so needs higher shock levels to completely kill it off (roughly an FC that is 60% of the CYA level instead of the more usual 40% of the CYA level, but this is unconventional advice on this forum).
Richard,

What does this mean?
If CYA is 100ppm, recommended shock level is 25ppm--that's not 40%, that's 25%.
If CYA is 60-90ppm, shocking should happen at 20ppm--that's 33% of 60ppm and 22.2% of 90ppm.

The only place on the "Best Guess" table where I see 40% is when CYA is 30ppm and the recommended shock level is 12ppm--but the range for an FC of 12ppm is 30-50ppm of CYA. I don't see where JHM says the CYA is 30ppm.

So...what does raising from 40% to 60% of CYA mean? Does it mean increase the recommended shock level by 50% (12ppm becomes 18ppm, 20ppm becomes 30ppm)?

Carl