frag starting to STN

[lorenz0]

well i was hoping this would never happen but i came home tonight and noticed that my purple slimmer sps frag is starting to STN. also on of my mille frags is showing some white spots and a few nights ago i noticed my emerald crag on the slimmer and found a black crab that looks like an emerald on the mille. Everything else is looking good and tonight i will be doing all my tests to see if i have any issues there. I have extremely careful with what goes into my tank and lately i have opened up to a few different sources. I traded with a member some colonies for some frags and the colonies all stn'd on him.

Corals aren't getting blasted by a powerhead, i am 2 days behind on my water change schedual but i don't think thats where my problem lies, i also have some cyano (sp) algae forming in a corner (probably due to lack of flow back there). but the slimmer has been growing great since i purchased it 3 months ago so this is a suprise for me. anyone with experience with stn'ing in coral please give me some insight to what your issue was so i can look into it in my tank.