In this case, since the RO water is being mixed into the remaining aquarium water that is NOT being changed out, it is not necessary to add any chemicals to the RO water. If you need to use straight RO water in some application, it may be desirable to mix in a little unprocessed tap water. If the pH of your RO water is too high, sphagnum peat can lower it. To raise the pH, a very weak solution of a strong base like lye would be a good choice. Use caution with concentrated solutions of strong bases and acids.
RO water is simple to understand.
It is just tap water most of whose salts have been removed. RO water is simple to manage. RO water is expensive to start. RO produces waste water you must dispose of. But remember, the waste water is very like your tap water.