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I am sorry your wife and 13 year-old are getting a rash from the tub. Take a look at this link and see if it seems more like a rash caused by a chemical reaction or by a bacterial infection. For professional advice, you should have your family see a skin doctor for a proper diagnosis.
From what I can tell, EZ SPAâ„¢ appears to be the following system:
EZ Pre-Startâ„¢ is used after a fresh fill to remove metals since it is a metal sequestrant.
Balance the water for pH, CH and TA.
EZ Boostâ„¢ is used initially as a disinfectant/oxidizer and is 99% Dichlor. One is told to use 2-3 ounces for any spa size.
EZ SpaOMâ„¢ is used once a week and is a combination product with a non-chlorine shock (probably MPS), a clarifier, scale inhibitor, and chemicals for water balance (pH, TA, CH). One is told to use 1 ounce (2 tablespoons) per 100 gallons. Note that one is told to wait at least one hour before entering the spa after using this product.
One is then supposed to test for Total Chlorine in a test strip to see that it is 1 ppm or higher.
After the above, for routine maintenance with an ozonator or mineral system one is told to use 1 ounce of EZ SpaOMâ„¢ per 200 gallons and 1 ounce of EZ Boostâ„¢ per 350 gallons once per week. With no ozonator or mineral system, one is told to use 1 ounce of EZ SpaOMâ„¢ per 100 gallons and the same 1 ounce of EZ Boostâ„¢ per 350 gallons per week.
Is the above what you are doing? The amount of Dichlor they recommend is about 12 ppm Free Chlorine (FC). The amount of non-chlorine shock with no ozonator is roughly the same. So combined, these would handle about 3-4 person-hours of soaking per week. The problem is that the non-chlorine shock is not sufficient to kill bacteria while the chlorine may not last for the entire week. Did you notice any pattern for when your family got the rashses relative to when you dosed with EZ SpaOMâ„¢ and EZ Boostâ„¢? If they tended to get rashes in the day or two after such dosing, then the high MPS and possibly the high chlorine were likely to be the problem. Also, please describe the bather load in your spa -- how large is your spa and how often were people using the spa (and how many of them) and for how long each time?
Non-chlorine shock (MPS) is irritating to some people so that is the most likely explanation, but let us know what you think based on the chart. You could switch to the Dichlor-then-bleach method and not need to use non-chlorine shock (MPS) at all. It will be a lot less expensive, but it will require you to add chlorine more frequently than once a week. If you are using the tub every day or two, then you could just add chlorine after you get out. We can help you with dosing, but let's first find out more about your current problem with the EZ products.