Go back to tap water or you are setting yourself up for a Calcium deficiency later on, and you DON'T want that.
That's a little scorch, nothing to lose sleep over. Next time your soil gets dry, just water until water runs out the bottom of the pot, and then water a little more- this will flush out whatever is causing the burning.
It is almost time to start feeding those plnats anyway, probably in another week. You'll want to start on a diluted fert solution for the first week.