Im of the opinion that you can keep anything you want without a reactor. I do. You'll need a Ca additive, like B-Ionic. But if you keep up with it.. maybe once every other day.. You should be fine.
i use tap water. our water has an incredible amount of calcium in it. i have had no problems as of yet, but it's awful early to be "out of the woods".
i am guessing that once i buy an RO unit, i'll have to start adding stuff.
i have some shrooms and a new green star polyp that seem to be enjoying themselves.