Thats what I get for having a crawfish in my quarantine tank

I know, it kinda defeats the purpose of a quarantine right? I had a snow white socofoli in there for about a week because it had some growth on its eye. It did fine, and my Cobalt Zebra had two white dots near his fin, and when I got home tonight I noticed what was becoming a white ring around his body. Any idea what that was? So I took him out and noticed he was very easy to catch. He was in the tank for about 40 minutes when I walked in and saw the corpse of my fish in my crawdad's grasp.
It my first big fish loss. I've lost mollies, guppies, and all that stuff when I was younger, but nothing like this. Oh well.