It may harm them in high dosage. So it depends. If your tap water aren't drinkable I wouldn't use them.. But mostly they're fine as they are. Ive been using tap water for years and my plants look great!
And i dont let them sit even for a minute..
Give them molasses and stop screwing with the water if the water where you live are good. (Don't ph them).
Tap water are great (here at least).
They provide suffice amount of calcium, magnesium and iron.
And I wouldn't recommend you to flush your soil. It's not that great for it.