Do you really want an explanation? This has nothing to do with it being a nano tank. I do not doubt it is well maintained but that is besides the point. Nitrates and phosphates will become a constant battle as the fish grow and get larger. Also aggression and territorial behavior will become worse.
Your stock is what id recommend for a 20g.
Im just informing, you have every right to dismiss my advice if you want to.
Not dismissing at all, but I researched plenty of these exact set-ups before I started and have seen many of them thrive, and this not my first saltwater tank , albeit it is my smallest. I am aware of the issues you have cited, however, in this hobby, I have learned that what works in
some tanks , may not in others and vice versa because of the many variables involved. You can ask five LFS's or tank experts for advice and get five different answers, etc, etc....I fully understand there may be issues down the road but I'll cross those bridges should they arise, but for now, everything is stable......