My cories have lost their whiskers. All of them. Just when I thought cories were the most resilient of FWF, the past few months a few have died for no obvious reason. My hardy survivors have all lost their whiskers. One of my old guys (easily identified because of his very solid black eyes) was the first I noticed. His mouth is a permanent 'O' from some sort of mouth rot. The other cories seem OK just whisker free. They seem stable (2 months or so) otherwise. The other fish in the tank are fine so far. Water. seems stable. No gravel change. Vac with every water change. 80% q2weeks. Thoughts?