Some or much of the information we've been taught about certain corals and NO3 & PO4 in relations to them is off. But first, someone correct me if I'm wrong but the paper for these automatic pre filter equipment, the filtration in microns I believe 20 or 40 microns. This is enough to permits certain particles to go through the mechanical filter and so breakdown to NO3 & PO4. Those using these devices obviously have to rely on a skimmer and whatever else works for them.
Now getting back to corals and information we've been fed about them. There are some members here that have easily 50 + ppm in NO3 & high PO4. And still have very successful Reef tanks. One soft coral case in mind is the carnation coral, dendronephthya species. We're using no mechanical filtration as pre filter, dry rock and a canister filter with dry Rock Rubble and a little old activated carbon for biological filtration, no skimmer, but on the discharge of return pump, we have a DE filtration, made in house. These corals are supposed to be in very close to Ultra low nutrient conditions. And yet they are not. Parameters are nitrates 15 ppm, PO4 is ?, we do water changes every week 2 to 5 gallons when possible with real ocean water. And of course RO water to top off.
Also with a good establish clean up crew and pods this helps with excess food.