[Snails will be the first to show you when something is not right. My experience matches with what Marc Daniels said. After my crash and tank re-start last year, I had a ton of something leaching from my rock, no doubt absorbed from all of my dying corals during the crash. Although the corals that were left looked good, I could not keep many snails alive. The ones I would set on the rock would soon flip themselves over. It was a few more months before things stablized, and things were fine after that.
Did you have snails living in the tank when you added these new ones?]