Phil, I do use methylparaben in my homemade mix, and the commercial mix I've been using of late (Repashy's Superfly), has at least *4* different mold inhibitors. Combined with the fact that I "irradiate" it until it boils, I'm confused as to why the "red stuff" still appears, but...as I said, it doesn't SEEM to harm the cultures, so I don't worry TOO much when I forget & use tap water. It just annoys me when they turn red.
Also Phil, have you ever read about how, when the soda companies first started using their "carbonation systems" (i.e. modern soda fountains), that whenever they cleaned the tanks, folks would complain about a "bad taste" in the sodas, however, when they left them be (i.e. uncleaned...natural organisms allowed to grow), folks relished the taste? I forget where I read/heard this, but I know it was a long time ago (probably in my teens when I worked at such establishments that used these systems)...I always kind of wondered if there was any truth to it, or if it was just another "urban legend". Food for thought, in any case. (And I thought it would pique your curiousity.)