Bpb,
Wouldn't that be as simple as your nutrients have no place to go because your clean-up crew and fishes remove any sign of algae before it can uptake nutrients? The skimmer will remove as much DOCs before they break down into nitrates and phosphates but once they are broken down, it does nothing. If you are still getting nitrates at really high levels without any algal growth, you've either limited one of the things it needs to grow, carbon, nitrates, or phosphates or your crew/fish mow it down so fast that it can't be used up.
To me, that indicates to either allow some algae to grow to uptake some nutrients, do more water changes, run a fuge/ATS, or employ a nutrient removal strategy specific for the nutrient, such as carbon dosing/sulfur denitritator for the nitrates or aluminum oxide/GFO for the phosphates. I will say my nitrates were in the 20 ppm range for 3-4 months with no ill effect to the acros other than they were a little browner than I prefer.
I still think all your recent deaths were due mainly to AEFW... not anything to do with your system. But, that's just off of what I remember of your system.