premixing and storing Five Star Star San sanitizer solution

While casting about for reasons why the witbier wasn't doing what I thought it should, I wondered whether there might be a problem with the way I was using the Five Star Chemicals Star San sanitizer. I typically mix a few gallons, use what I need, then store the rest. It occurred to me that it might lose its efficacy when stored so I went to the company web site, found the contact info for the tech support and new product development guy, and shot him an email. He replied very quickly (and gave me permission to quote him):

If you are going to do this, use DI water to make up the solution. Tap water contains metals that can cause the solution to go bad over time. If you use DI water, keep the solution out of UV light and extreme temperatures, the solution will not degrade. People then ask the question, "How do I know if it is still good?"

Answer: Is the pH below 3.5 and the solution clear. If the solution turns cloudy, it is starting to go bad.

Thanks
Jon Herskovits
Five Star Chemicals

So, I haven't been using deionized water, but neither has my solution grown cloudy in the matter of days or weeks that I've stored it. Mixing Star San is very easy, so there's really not much reason for me to make more than I need other than that I do like to keep a spray bottle handy. I'll have to check the pH and see how long it keeps with my tap water. I haven't seen DI water at the couple of grocery stores I've checked.