it was about 15 years ago my dad had this gadget so could be mistaken, I swear is was to remove what I said though.
clorine is a big killer of fish and that need to be taken away, hard fish can take it the others can and will die should this not be removed.
Rain water is a good option, never heard that the water needs minerals put back in it after ro, ph buffers maybe, and declorinator, loads of poitions you can buy, I can also supply them mail order