Since you said fish owners opinion count, I am going to make this reply.
As a fish keeper, I use tap water treated with aquarium water conditioner. It is recommended to do it because it can avoid problems in case of you can't get the source of the water needed for a single water change. Fish like to have the same type of water with the same PH, same hardness, etc. Sudden large change will shock them and might result in deaths.
Saltwater aquarium owners often use RO water. But that's a different story.
I will always use local tap water as long as the tap water isn't extreme in hardness or PH.
