I've got the exact same fear as many of you. However, the guys at aquariumfish.net SWEAR that if you change 20% or less of the tank's water at a time, you can put straight tap water in (no treatment before or after) without harm to the fish or the bacteria.
I'll let you read what they have to say yourself. Please, don't take my word for it!
I've had the guts to try it once this week. So far, everything is still alive.