There are lots of produts out on the market. Some work, others do not. The best thing, of course, is RO/Di. Unfotuneatly, most units, as I am sure you know, are expensive.
What I would do, buy water from a local store. Even distilled water to start with and then use tap water later for changes.
Tap water purifiers do not do much in the way of removing all the metals but in a FO tank, you don't need to worry too much about them.
I have used stuff called PRIME. It works very well and will not make your skimmer go crazy. It would be what I would suggest.