I'm in the UK and I've always used tap water. My 21 year old has never complained.
I agree with Susan - if the tap water in your area is fit for humans to drink, then it's fit for Corns.
Tap water in the UK contains some chemical treatments (depending on your local water authority), but I've never seen them linked with any harmful effects on Corns. By the time the water reaches your tap, there are no more than minute traces left anyway.