I'm new to the forums. One of my kuhlis is not doing so well

When I went to feed the fish today I noticed that he stayed in his cave and wouldn't look for the food. Then I put some food in front of him and nothing, he didn't look interested

So, I decided to improvise a small hospital tank (my actual hospital tank is holding a goldfish right now that, of course, wasn't in the same tank as my kulhi) and move him in there. It's just a 17.9 liter plastic container with a little cave and an air stone.
When I caught him I noticed he had like a colorless patch on his back, as if the scales had come off


Any ideas on how I can treat it? I haven't started him on anything because I've never treated scaless fish and am not sure what kiind of meds I can use
Please, help!
The water params in the main tank were ammonia 0, nitrites 0, and pH 7
Thanks
Chris