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I've been having the same type of trouble with my skin. Behind one knee, I've clawed it to pieces, but after a little time, I've almost got it healed again. I think my problem was a very high PH. I'm doing better now since the PH is closer to where it should be. I was also using too much Chlorine, so I've tamed that down a little. I'm fixing to try the EcoOne that JohnnyK45 suggests to see how that works. It should be here in a day or two.
general rule of thumb is right around 1.5tsp per person/per ~20min. use...obviously if the amount of bodies increase or your soaks are much longer you will need slightly more. If your a regular user 4+ times a week you could shock on a non-use day...I have a couple that are customers going through the exact same thing right now..MPS was wreaking havoc on their skin so they switch to small amounts of chlorine and so far they have had much better results. Also to state the obvious make sure your pH, Alk. and Calcium are within balance, mis-balanced water can also cause some itching/scratching/redness