I'm speaking from personal experience and having similar issues. I went through a period where I lost a 12" massive stylaphora coral. Supposed to be easy to keep SPS, up and died within a week. I lost a birdsnest the same week. But other SPS was fine. I thought a dip of the skeletons would yield critters, but yielded nothing but algae. . .
I decided to do an ICP test, thinking there has to be something off, a stray heavy metal, maybe a screw in the sump or something strange and weird. . . The ICP test was revealing just not in ways I hoped.... It told me my red sea phosphate tests were crap. . . Always showed .16. Not great, but shouldn't kill SPS...
The ICP Test showed no signs of any heavy metals, in fact I was lacking in all trace elements (they were all undetectable). Which was a different problem, but, the biggest issue was, phosphates were detected at .66 not the .16 I thought they were at. . .
I worked to slowly lower my phosphates over a 2 month period from .66ppm to .05 ppm. Now, my SPS, no matter where they're at in my tank are doing amazing. My LPS hammers are growing new heads like crazy. I have Alveopora, supposed to be hard to grow, growing fast and beautifully. My Acropora are continuing to grow and looking great.
My chalice that had tissue dying off all over the coral, regrew new tissue and sealed up tears in it's skeleton, now, you would never know my chalice had been rotting.....
I too thought maybe it was lights, but, phosphates were the big issue for me. Our test kits don't do well. I got a Hana phosphate checker after the phosphate test, and it's saved me many $$$ in corals. Sometimes it's the simple stuff. I dose trace elements now also. (Red Sea A, B, C, and D) a couple times a week. I just detected phosphates rising back up to .15 this past week and noticed one of my acropora having tissue recession. I reduced phosphates down to .05 over a 4-5 day period and and the recession stopped and it's showing new growth on its tips.
My opinion is don't dismiss the simple stuff. Do an ICP test to confirm your real phosphates or get a Hana checker if you don't have one and verify they're really .3...
The .3 ppm phosphates and almost 10 dkh alk, combined, could be a recipe for SPS killer. Also make sure your DI resin is changed often enough....
Your water must have too much or not enough of something... Lights could be an issue, but usually the lights will affect coloration and longevity of the coral, it shouldn't kill it within a few days. . . To me that's a sign of water issues. Salinity could be off (too high), phosphates are for sure too high, and alk is bordering on dangerously high. My GSP won't open if my alk hits ten, along with my toadstool. Both completely get mad when my alk hit above 10 DKH.
I'd be careful with the dosing... My SPS are doing great at phosphates of .05ppm and alk at 8.3. Calcium is around 525, and magnesium just over 1500.
Just some random thoughts! Hope you find the issues! If I had to choose between a Hana ULR phosphate tester and an ICP test, I'd probably go the Phosphate tester first.
