Mike up,
Not to horn in on your discussion with Altum Nut, but WCs are necessary....maybe not daily, but replacing old good water with fresh good water is indeed crucial.
I can see why you think as you do, but there is more to “good” water than the Nitrogen Cycle. Your TDS levels, KH, GH, Potassium, and more also factor into healthy fish as well. For example.....if you add water to your good water that evaporated, you will likely have changes in mineral levels as well as TDS levels. Such changes are surely going to be detrimental to the fish in the long run. In addition to my Discus tank, I even do WCs with my community tank of Gouramis, Tetras, Platies, and Angel in which no Discus are in. Water changes are considered to be required for virtually all fishkeeping these days. I have done fish for decades and remember the old school ways of not advocating WCs..
. Lastly......if you do no WCs, your NO3 levels will rise down the road.....guaranteed. WCs are the ticket to a good foundation of fish husbandry. If you wait to do WCs until you have ultra high TDS levels or Nitrate, it will be harder to get the water to desired parameters, whereas if you routinely do WCs, such drastic conditions are much less likely to occur and your fish will have the greatest living environment.....that of stability/consistency. Remember.....that what water is good today with life in it will surely degrade or become worse if not replaced.
It’s like this...sort of......
Take a shower today and you’ll feel great. Tomorrow w no shower, you’ll feel good. Day after tomorrow w no shower, you’ll maybe feel ok. What will you feel like after two weeks wo a shower? Fish are similar in that water that’s fine today will be much less so in a two weeks or a month wo a change of some of their water. Sorry for this goofy analogy, but when you said “Maybe I’m missing something”, I felt compelled to state my thinking in two different ways.
I hope I did offend anyone or improperly state reasons for WCs, but sharing like this is what Forums are for as I’ve learned much from them and will continue to do so. Btw.....HNY to all.