You definitely need to dechlorinate the new water before adding it to the tank. If you keep up with regular weekly water changes, you shouldn't have to worry about different pH levels (or whatever) because it won't have a chance to change much between maintenances.
However, do I understand correctly that you've never done a partial water change in 6 years? Or do you completely change the water now and then? The reason I ask is because you want to be careful about changing their water conditions too rapidly. This could cause stress to the fish. So, you might want to go with 10% weekly water changes for awhile 'til the tank water conditions match the tap water conditions more closely, then keep up with 20% changes weekly.
Hope this helps!