A skimmer, corals, and some ceramic plates. I have a low fish load currently but I can't keep my PO4 up and have to dose it at times. Nitrates bounce around (according to my ICP tests) but I don't care about them. They aren't dangerous to fish to or corals so I am not sure what the fascination is with nitrate these days.
I have had starfish almost die from high nitrates. And I have seen irreversible gill damage on clowns from high nitrates. But we are talking in the red on the test kit. Also, there's the whole algae thing. Low nutrient systems are easier to manage in general. If you ever try to keep a macro algae tank you will find this out quickly. Macro algae will never out compete nuisance algae with high nitrates. But on the flip side, if nitrates are too low, macro algae will go sexual and die off, particularly caulerpa.