Did you test your nutrients to see if they were in fact too low?
I did test for po4. It’s 0.03 ppm. I don’t know how to interpret it, low/normal?
But the thing happened when I added more fishes in the system (3 anthias, powder blue tang, 2 wrasses, 2 other small fishes). Since adding too many fish in a short period of time so I decided to add bacteria in the bottle (bacter 7, overdosed with 4 cap~20mil per day). Maybe due to the amount of bacteria added, the nutrients suddenly dropped and affected the corals. Well, that’s how I explained it not base on test result.
I did water change and carbon to make sure if there is any toxic element (something dead, animals drop in the tank,...) they will removed. Then I set up an automatic feeder that feed pellet 4 times per day (still feed frozen 1 time). And cut back the bacter7 to 1 cap per day. Also lower the light intensity to 45% (from 70%).
I don’t have any case of rtn/stn for a couple day now.