Dan,
Keep the ULN system, but feed more variety of more food, add more diversity to the tank. I think Biggles is of a thought that 1 fish per 10 gallons might be the low scale for colorful acros, having a belief that more fish, fed well with variety, with an aggressive removal system=colorful sticks
I personally feel that my 12 in 120 is perfect considering the size of my fish, they are huge and poop quite a bit, so every 3 days without fail, I have to swipe my glass. My skimmer goes nutz, my corals extend with feeders at the tips. I know this makes them very happy

While I do test weekly for nutrients, I don't feed to much into them anymore, while I have never seen this system, while ran differently at times, ever test higher than 0.02 for PO4 and 2-5ppm for NO3. In fact, I too cannot register on any test kit anymore for either, but know every 3 days I will swipe that glass, so I know it is in the tank, but there are no types of algae present, at all... I think the skimmer is the heart, it skims constantly, another thought to me is that I get away with sand, my goby and cucumbers are amazing, so while BB ran tanks have an advantage of nutrient removal, I have perpetual removal with constant food source being delivered to my tank, whether it may be stirred up fish food, or sps food, all day long, sand in my water column, really only upsets me during photo time

I am not sure of your fish count, or your feeding to your system, but I would simply start by feeding 3 times at least per day, maybe even drop aminos every once in awhile. Or, I would dial down your C-Source and Bac dosing quantities, allowing for nutrients to rise that way, or maybe a combo of both.