A few quick thoughts from somebody who alters water params in one tanl regularly in a few other ftanks as needed to simulate dry/rainy seasons and who owns a portable RO/DI unit.
RO will remove a lot of things but not all of them. Most units have multiple modules which contain different media which should remove specific things. What modules one needs depends on the problems and desired end results.
So, the initial modules would be a sediment filter and an activated carbon filter in that order if both are needed, Th first one should be obvious as to what it removes. Carbon removes a variety of things to include these important to us ones:
Carbon removes a class of material called dissolved organic compounds (DOC). These compounds are produced by many different processes in the aquarium. The two major categories of organic substances that carbon removes are tannins and phenols. Tannins give the water a yellow-brown color. They are produced mainly by the breakdown of plant material, but there are other sources too. Phenols give a fish tank that characteristic “fishy” smell. Carbon can also remove chlorine and some heavy metals through other processes.Carbon does not remove ammonia, nitrite or nitrate from water. It also does not have an effect on water hardness or alkalinity.
from
https://www.drtimsaquatics.com/aquarium-hobby/general-articles/activated-carbon/
(If your water contains chlorine.chloramine, you need to use the carbon module because chlorine will break down the RO memberane.)
Next inline would be the RO membrane module. This removes a lot of stuff, but not everything. It wont remove everything but it does remove a lot. However, there is one more module one may wish to include and I do so in my small 3 module system, And that is a DI module. DI stands for deionizing and that means the removal of ions. Ions are electrically charged atoms or molecules found in water that have either a net negative or positive charge.
The Ions we all know would be Ammonium NH₄⁺ Nitrite NO2− Nitrate NO3-.
Basically, a system which flows input water --> sediment module -->carbon module-->RO module-->DI Module should produce almost pure water. There are more module one could have if they are needed. I do fine with carbon-->RO-->DI but I have to backwash my init to flush the sediment it catches. It is not a lot but enough that I would get a 4 stage unit the next time were there to be a next time. I have well water and do not use dechlor. But I need soft acid water for one tank.
So, your solution may or may not be making water to specific parameters. it is not simple nor easy to do. The chemistry is all intertwined. Hardness, pH, KH, GH etc are not isolated from each other in terms of what they can affect.
The best advice I can offer is the less meddling with parameters that one has to do the better.
Once you change your parameters, you will also need to change them for the water change refill water.
