Except for some extreme circumstances there is absolutely no need for an RO filter for water change water with freshwater thanks.
Only thing you probably need to add is something to neutralize chlorine, otherwise straight from the tap will be good.
Extremely unstable, wastes huge amounts of water that would be better off used for water changes, wastes money. You'd be using stripping all the hardness and minerals out of water that may not be perfect but is probably very usable and then spending money to put them back in so your fish can survive in it.
The ONLY time an RO filter would be used regularly for a fishtank is for a marine tank and the majority of that is for topoff water.