Mustard algae experience

[CarlD]

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

[chem geek]

You should move my post (or the part talking about FC % of CYA), yours and this one to The China Shop.

Ben and I disagree about shock levels so if you are using the Best Guess CYA chart the yellow/mustard shock level would be 50% higher as you describe. However, in terms of the amount of active chlorine (hypochlorous acid) in the water, the FC as a % of CYA tracks that better than the Best Guess CYA chart and in theory would provide the same rate of algae kill for shocking. Obviously, this becomes unwieldly at high CYA levels and in practice lower FC levels will still kill algae faster than it can grow though it will probably take longer to clear the pool.

The min. FC level for inhibiting yellow/mustard algae seems to be about double that of green algae, however, so I don't think the 25 ppm at 200 ppm is going to be sufficient to kill the algae. On the other hand, if your CYA is that high, the proper prescription is to to a partial drain/refill to lower it significantly. At shock levels, increasing the FC by 50% in the chart results in roughly doubling the active chlorine concentration so your "use 50% higher shock levels" would be reasonable.

Richard