Rope fish health

I recently got a rope fish and his health seems to be going down hill. I was
told they liked brackish water and that is what my tank is I have read all
about what they eat and I cant seem to ever see him eating. Are they a fresh
water fish or will brackish water keep them ok? I don't want him to die..

The other fish in the tank are 1- little Neon 5-silver tip sharks 2 - puffer
fish 2- cory cats and 1-
pa?o?umos"sucker fish"
Any help ????
Thank You Troy