Regarding your ro if continue to use ro water you'll be fine with water changes, there's still ways of introducing phosphates and nitrates into your tank isn't ro water is just one variable we control another is food and dying matter and also rock sand and foreign objects plus alot more I'm sure lol the trick is to introduce as little as possible so it doesn't show on our tests once your system uses what it needs we don't want any excess or those nutients are quicky used by pests algae and other problems we don't want.
cheers