Alk swings tend to nick the tips more so than the whole coral, it might not even be alk swings, thats just the most common cause. Again, turaki, echinata, suharsoni, etc. tend to be pretty picky. Have you had any ugly temp excursions?

It might just be your angel also, hard to say. If your going to do some of the trickier sps you will just have to get a high end alk kit, I like Lamotte because you can buy refill kits for about 13bucks vs buying an entirely new $35 dollar Salifert kit, though both work just fine.