Question about shocking

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Belldiver

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Sep 7, 2007
344
Lafayette, Louisiana
I'm currently trying to rid my pool of a small algae issue and have a question about "shocking". My CYA is ~70-75 so according to the Pool Calculator my shock level is 20-21. When I bring the FC level up to 21 and it then drifts down into the 12-17 range during the day/night, am I still "shocking" the pool (just a little slower)? Or is all that time below shock level being wasted? At what FC level (relative to CYA) are you still gaining ground and killing algae? Or are you still gaining until it gets down below minimum FC levels?

Thanks...
 
Whether you have a little algae or a totally green swamp, algae is algae. For shocking purposes, unless you bring your pool to a shock level of FC for your given CYA, you aren't shocking anymore. Best to bring it there and keep it there at 21 until your FC holds overnight. If your shock level is 21 and your FC falls to 17, you aren't shocking anymore.
 
The recommended shock levels are to get a reasonably fast killing and clearing of algae from the pool. If the level drops somewhat, but not too much, lower (say, no lower than half the shock level), the algae is still probably getting killed faster than it can reproduce, but it will take a lot longer and there could be pockets of algal clumps or biofilms where the algae will be able to grow faster than chlorine can kill it. It is best to MAINTAIN the higher shock level to get rid of the algae more quickly, yet still be safe for the pool.
 
Chemgeek's well thought reply is worth re-reading. It's not that something abrupt or magical happens at a certain shock level of, say, 21. The same thing happens almost as well @ 20 or 22. However, the same thing does not happen just as well @ 12ppm or 45ppm

Too much FC is frequently bad and too little FC is almost always bad. Chemgeek and others have developed these numbers as markers which tell us approx. where we need to be.

If you come across a situation where two numbers are fairly close for FC, you can feel pretty safe choosing the higher one because the process will proceed a little faster and you are still within a margin of safety.
 
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