Whats up?
388 is on the high side for tap water but it should be ok. I use 350ish tap water for my seedlings. If I were have to guess it was probably the very low PH. Distilled water lacks minerals and such that the plants need. Its good if you supplement those minerals but tap water makes it much easier because all that shiz is already there.
Maybe you have reallly bad chlorine levels but it was probably the PH...

Just my 2 cents.