I'm not going to add to the ongoing debate, but I'll put my thoughts in to help the OP. I think your corals are suffering from a lack of available nutrients. Zooxanthellae driven photosynthesis is a great way to sequester carbon from CO2 which is always present in our tanks, but nitrogen and phosphate must come from food. For many the association with higher detectable levels of nitrate and phosphate correlates with the ability of the coral to sequester it, hence many will say you want at least detectable levels of these nutrients. In other systems undetectable levels is still enough to help the coral thrive. It really comes down to a dynamic interplay between different conversions of the nutrients in various biosynthetic pathways and that is why a one size fits all doesn't work when it comes to nutrient levels and reefing. Bioloads, biodiversity, feeding habits, nutrient import/export all play largely into how a tank will show nutrient levels on a test kit. Keeping in mind that phosphate does inhibit the calcification reaction, little is known about the bioavailability of dissolved phosphate and coral.
Anecdotally my tank has slower growth but more color when I dont run GFO and have a little algae here and there. When I run GFO and am algae free sometimes the growth is faster for a bit, but I have correlated this with increased sensitivity of my coral and sometimes coral death. If I were to put forth a hypothesis I would say the coral are growing off nutrient reserves when I tip my tank towards ULNS, and when the nutrient reserves run out they are more sensitive and less resilient. Other people do just fine running ULNS all the time, but most likely have other factors at play that aid in coral receiving the necessary nutrients.
