Been there, done that, waiting for Brett on the T-Shirt!
Ya, sorry, I won't be much help here either, but I too feel your
pain and frustration.
To this day I have no clue what caused my die off. After a few hefty water changes it seemed to sort itself out whatever it was. It certainly was not anything I could test for. Was not the parameters, light, flow, or pellets, all of which are completely unchanged to this day and all my corals are more or less back to their former glory (growth and colour). Go figure.
Sure I would like to know what happened, but I am not going to lose sleep over it. It is what it is and trying to chase it is just an exercise in complete and utter frustration. Do your due diligence and test what you can, water, equipment, etc. and rule those out. If all of that checks out, then there is a variable that you just can't account for.
If I had to guess, my money is on sickness. Animals get sick. Animals that are clones of one another living in a colony are at a higher risk because if one gets sick, they will all get sick very very quickly. Some animals can fight off the illness, some can't. Even in this scenario there are so many variables. As suggested already, an ULNS is a virtual state of starvation for many SPS as we try to keep them alive precariously on the edge. On one hand we strip them of as much nutrient as possible while on the other hand dosing just enough of the nutrients that
WE want them to take up so that they look pretty. This in
my personal opinion is not their natural state of existence. Out of all the books and programs that I have read on SPS, the conclusion that I have drawn for myself is that some (or many) SPS corals are a lot darker in their natural habitat. They have the exact amount of zoozanthellae that they need to flourish. Some will take up more some less. The most common colour of zoozanthellae is brown! If a particular SPS needs or wants more zoozanthellae to survive, great, it will store more and thus get browner and is perfectly healthy and happy now. However, we as hobbyists don't want that. Instead, we try to get them to expell as much brown zoozanthellae as possible in order to bring out their colours, even if this puts them into a state of starvation.
The bottom line is it is a delicate balance that can tip either way.
Oops! Sorry for the essay/verbal diarrhea.
