1) Remove the catfish or add target fish/dithers so the catfish doesn't just concentrate on the datnoid. If that doesn't work, get rid of the catfish.
2) Use melafix and salt to heal the tail.
Yeah just get some dithers such as Silver Dollars, I'm using them right now as dithers and they are the only dithers that I enjoy having in my tank, They are with my Silver Dat right now and are perfectly fine. As for the tail get some pharmaceutical salt and Melafix and you are good to go!