*Can* you use tap water? Sure. *Should* you use tap water? Absolutely not. Tap water contains a lot of trace elements and chemicals that you have no control over, and some can be difficult to test for.
I will admit that I used tap water for the first two months of my tank. Since then, I've been using the Glacier water machines outside my grocery stores, which is RO
, and double carbon filtered.
Sure, it's a bit more work, but it's worth it. Water is clearer, cleaner, and I can control all the water parameters much more easily.
Plus, some communities' tap water has phosphates. I have a customer who has been using tap water for her freshwater tank and it's always been cloudy. Her blood parrot cichlid had some fin rot and she couldn't figure out why. Turns out her tank's phosphates were off the charts. And read off the charts almost instantly rather than "wait three minutes for color to develop". A PWC
water and the cloudiness started clearing up.
Moral of the story, control what goes into your tank.