Water conditioners like Nutrafin AquaPlus neutralize chlorine, chloramines and heavy metals. Yes, that is important but not the only reason for using RO water. One reason for using RO water is to make sure you are starting from scratch and then adding elements back to the water with your salt mix. If you are talking about using water to top up the aquarium. I would use RO water and then add a buffer to make sure you are not reducing alkalinity or KH, you could also use Kalwasser/ calcium hydroxide. With a small nano tank buying RO water is not that expensive or much of a hassle. Although I would probably invest in a RO unit. Hope this helps!