Thanks Evan for clarifying what I meant which was to use a cumulative total of 20 ppm FC of Dichlor. I didn't mean to use it all in one shot! Shows what happens when I've got a concept in my mind and only some of the words come out in a post.
It will take 3.6 pounds of Dichlor total (cumulative) that you should use, but as Evan says, not all at once. Here's a plan I think everyone will be happy with:
1) Add 2.5 pounds of powdered Dichlor. That should raise your Free Chlorine (FC) level by about 14 and the CYA level by about 13 (Evan, these numbers use the 12,000 gallon pool volume given in an earlier post).
2) Keep track of the chlorine level and if it dips down towards 10, add another pound of Dichlor which will add about 5.5 ppm FC and 5 ppm CYA.
3) Keep track of the chlorine level and as it dips down towards 10, add one gallon of 6% bleach (Clorox Regular, or off-brand Ultra) which will add about 5 ppm FC (and no CYA).
Repeat step 3 (that is, using only bleach) until the algae turns from green to blue-gray to gray to cloudy white to clear. Run the pump 24/7 and brush the sides of the pool periodically (carefully, using a soft brush).
Your local pool store is sure to have Dichlor. It is any of the following brands and is normally used for spas (but there is absolutely nothing wrong with using it for pools when you want to add both chlorine and CYA in a hurry):
SpaGuard Chlorinating Concentrate
GLB Granular Dichlor
Rendevous Chlorinating Concentrate Granular
Leisure Time Spa 56 Chlorinating Granules
Robarb Granular Dichlor
Robarb Sani-Spa
Most any chlorine that says "Dichlor" and is in granular or powder form (not solid tabs).
Richard
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