You're here now and asking for help, so that's a great start. I would keep up some daily or every other day water changes to keep the water extremely clean. Also, it wouldn't hurt to dose Prime once a day for the next week. Make sure you use enough to treat the whole tank, not just the gallons you add.As far as feeding, I'll go get some frozen foods tonight. Again was told live shrimp was the best for him. I'm aware ignorance is not an excuse but had expectations of the store having fairly good knowledge lol. Sadly he was the better looking eel they had there.
As far as the ghost shrimp go, they are likely the cause of the ick. Live aquatic feeders are notorious for spreading disease.
As for finding a food that your fire eel will like, I found it was best to put a little bit of everything in. Just be ready with the siphon to get all of the uneaten stuff out. I recommend trying frozen bloodworms, frozen mysis shrimp, live red wigglers, and some cut up market shrimp. Just put all of that in and see what the fire eel wants to eat. After you find something, use that to fatten him up and eventually try to get him eating other stuff.