I started losing corals a couple months ago but didn't think too much of it because it was only one or two, and I kinda thought something was eating it or something (Not impossible since I have a decorator crab, a fire shrimp that likes to rip food out of corals). Now I'm seeing 3 or 4 more corals with the polyps hanging off their skeleton. I tested my water for alk and calcium (I'll test the rest when I have time- I've had stable undetectable of nitrates ammonia and nitrites for a while, and I have a refugium and a bioplastic reactor and a skimmer to keep them down, so i doubt that's the issue). They were both a little low (for reef tanks), with calcium at 400 ppm and alk at 7dkh (I'll be adjusting my dosing schedule to bring those back up). I'm starting to think it's something bacterial or something, because itt's been bouncing from one coral to another. Half of my corals are doing well, showing consistent growth and growing new heads, then they start closing up and then bail. It's happened to my micromussa, my bowerbankii, a head of my duncan, my cynarina, and is starting up with my button scoly- any help would be greatly appreciated.