Confused over mustard algae

[woodyp]

"Yel/Mstrd Min" is 15% of the CYA level"

Ran across that in reading up on killing mustard algae, and I have no idea what it means or what it is worth. I understand the shock value of 60% of CYA value but have no idea what good the 15% minimum as referenced to mustard value represents. When would that be used and what for?

Related question: Why is it recommended to bring the pool to normal shock value for 24 hours before proceeding to the "upgraded" level for mustard algae? Why can't you just go straight to MAXIMUM!

 

[chem geek]

It means that if you haven't completely killed off yellow/mustard algae everywhere in the pool (including behind light niches and under removable ladders) and on anything that would get into the pool (polls, swimsuits, etc.), then you'd need to maintain an FC that was 15% of the CYA level to keep it from growing in the bulk pool water. Yellow/mustard algae is about twice as resistant to chlorine as regular green algae. Such a high chlorine level is often impractical since its more costly to maintain so it is better to completely get rid of the yellow/mustard algae after which one can maintain the more economical FC level that is 7.5% of the CYA level (or 5% for saltwater chlorine generator pools).

When shocking a pool with yellow/mustard algae, you could go straight to the maximum if you wanted to. I'd recommend getting the pH quite a bit lower at first since the pH will rise a lot when using a hypochorite source of chlorine (it will come back down when the chlorine level drops after you are done shocking).