CPAPPIEDENTIST is right.....but,

You can add tapwater, but you must allow it to set for 48 hrs to allow the chlorine to vent off. I use a small secondary tank filled with tap water that automatically keeps my primary tank with minnows full when it evaporates.Not that big a deal with temps right now, but as the temps climb upward, the evaporative proces will re-claim quite a bit of water in a days time.