It's difficult to say sometimes exactly what is being removed or replaced with a water change. IMO, it's too difficult to really tell exactly what is being depleted or building up in your system as not all water parameters can be tested with hobbyist grade test kits. Alk, Mag, Ca, Nitrate, Phosphate...etc may all seem spot on with whatever dosing you may do, but trace things can build up or deplete without you knowing. IMO, unless you are doing the Triton method or sending water samples out periodically for a full blown lab test to track everything, than water changes are a good method of keeping everything balanced. Although I monitor and dose for major and some minor elements, I don't at all think that my water is absolutely unchanging over time.
I did go a long time with minimal water changes, averaging maybe 5% change a month for over a year, and everything seemed okay. But I then did a few 30% changes over a few weeks and all of my coral opened more, grew faster, and got better colors. All of my testing showed no changes in any perameters I test, but the change in coral's aparent health and appearance has convinced me that something good is happening with water changes. I now do 25-30% change a month. With a system like yours, you may be okay streaching out your water changes. I'd just watch your tank for any negative effects along the way. If you did do less or minimal water changes for a while, maybe try what I did and do a few good sized water changes every week for a month or so and see if your tank changes in a good way, or no real change. That would tell you if it's having a significant impact on your system or not.
