Ich man

I have had a hot tub for over 30 years and used it daily without problems mostly using borax and occasional additions of a clorinator or household bleach. In February 2 this year my old hot tub finally failed and could not be repaired so I purchased a new 265gal one. This came with a number of chemical additives and the initial fill instructions were: 9 tablespoons of alkalinity increaser, 90ml Silk Balance and 1.5 tablespoon of Spa Sanitizer chlorine powder. The temperature was set at 104F. Since I had used borax successfully in the past I added 1 cup of borax. The test strips indicated low chlorine so I added 1 more tbsp of chlorine powder. on February 14 I started to have problems with a skin rash and itching on my legs. This got worse and on March 20 I emptied the tub and cleaned the filters, refilled as before but with 1/2 cup borax plus 1 cup vinegar. I add weekly chlorine but test strips always indicate low to no chlorine and the water is still causing itching on my skin. what am I doing wrong?