where to put the sock full of CYA?

CYA in the skimmer basket is just fine as long as it does not completely block the water flow into the skimmer. This is true even if the pump is not running. However, you should run the pump for 24 hours after adding CYA anyway, so that doesn't usually come up.

It isn't good to leave trichlor in the skimmer when the pump is going to be off. Trichlor is much more acidic than CYA. Left in the skimmer trichlor will build up very high FC levels and very low PH while the pump is off, which can then damage the equipment when the pump turns on. CYA is not nearly that acidic, and it doesn't affect equipment the way chlorine does, so it is not a problem in that sense.
 
What I normally do if I need cya to rise, is to take a cya level reading, fill 1 plastic cup with CYA, dump the contents of the cup into a sock, tie the sock with both a tie and rubber band, put in skimmer in the morning just after pump starts, wait 3 days, remove sock (all the cya should have disolved), wait ~2 more days and read cya level, if still too low, repeat process until cya is where I want it. I'm not aware of any issues with leaving the sock in the skimmer....just make sure to have a sock that doesn't have any holes and keep in mind that at least for the first few hours - there will be at least a ~0.1 drop in pool pH for my size pool - but if done at the start of the day is normally easily corrected via airation by end of day. I wait for at least 1hr after putting sock in before letting anyone swim - although that is probably more a matter of personal preference. I don't run pump continuously for 24hrs after adding, only ~12hrs which is usually enough given our water temp is ~88 degrees most days and the skimmer gets good water flow.
 
I just added 4 lbs via an old sock in front of the return. It took less than a day to dissolve. Add the CYA, tie off the top, stick it in front of the jet, put a brick on the top to hold it in place. That's it. If you want to rush it, squeeze the sock every so often.
 

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