If your tap is hard and/or heavily chlorinated, it's going to mess up your humidifier eventually as well. It's a pain but I run only distilled water through my humidifier. I hate using tap for any aspect of growing because it's really not good for the plants. As many old-timers around here will tell you, tap water in general sucks. Also when you pH, if you're using paper make sure you let it sit in the water for at least 30 seconds. Because the tap is unbuffered it takes longer to get an accurate reading.