My biggest concerns for most tanks are organics that don’t bind to GAC and the commercial resins folks use, and there is little to no dats on which organics any of these methods bind in a reef tank.
Accumulations of some inorganic ions are also a concern, especially if you do not get quality icp testing, but even if you do, water changes might be the best way to reduce them.
What would be an example of these organics that you are concerned with?