Due to the way water chemistry works it is not possible to keep a fish in pure reverse osmisis without remineralization and carbonate hardness (KH) buffering long term. You will eventually experience pH crashes and swings which will kill your livestock. R/O is the perfect basis for creating the ideal conditions for your particular fish species however. You will need to remineralize the water (example, kent R/O right) and then buffer it to a sufficient carbonate hardness to maintain the desired pH which depending on the amount of remineralization usually hovers around 7.0 with R/O. It is however possible to adjust the pH prior to locking it down with a kH buffer using products such as API pH up and API pH down.
You will need a very basic knowledge of water chemistry as it relates to mineralization. You will also need pH, general hardness (gH) and carbonate hardness (kH) test kits with accurate liquid reagents. None of this test strip garbage.
If that all sounds like Klingon PM me and I will explain it better. Don't want to type 20 paragraphs right now if you already know what I'm talking about
HTH