I think it's a combination of things and sometimes it takes a while for these things to affect the corals. I went through a series of TN issues that would randomly hit pieces, while others seemed unaffected. When they have that dark, dry look to. them, it's a combination of starvation (usually low prolonged nutrients), too much light and mini swings with alk and/or nutrients. I also agree with Smite that heavy metals are usually detectable during these events and low pH just pushes them over the edge.
Dosing a half ppm to 1ppm of NO3 nightly helps and getting your Po4 closer to 0.1ppm gives you a better buffer.
If you're not on FB, join just for the RM support group, Andre and others are always there to help other shiners. Also buy his color guide, I think it's only $5? I used to keep low residual nitrate levels, thinking that keeping it closer to NSW would be okay, but our tanks are so different than the natural reefs and we need to keep a "buffer" of nutrients available at all times, so I have a minimum of 10:1 (or slightly higher) nutrient range, nitrate to phosphate. A lot of people go even higher, which is fine. Don't get caught in the Redfield ration BS (yes, it's completely BS in regards to our tanks and has nothing to do with coral zooxanthellae), but do keep in mind that ranges regarding N
are necessary, just not what people think.
Hopefully the ICP will give you some insight, but don't let that be the only deciding factor in how you approach this. Old school observation will always tell you everything.
Good luck and keep us posted. Smite and I are fellow shiners, so reach out if you have any questions?
