There are people that have good luck with tap water, but there's a lot to be said for peace of mind. Especially when you're dealing with sensitive livestock worth thousands! Also, when you're having a problem, it's nice to be able to eliminate source water as the cause.

Very good point. You also have to know what is in your tap water and know that not all tap water is the same. Also you are adding way more phosphates from feeding then you do from tap water. My biggest worry would be metals from the pipes. Things that i can't test for.