Fire eels are nice fish. I'm pretty sure they shouldnt have any salt in their water. Most eels in this family are shy though, so its normal that your fish is shy. They generally come out at night more. Maybe you could get a dimmer light to see him at night or something. I heard moonlights would work with nocturnal fish like that. But as said, you could probably get him out more if you fed him more of a variety of foods. The best foods for this eel is meaty fishy foods, such as mussels, shrimp, whitebait, and other types of fish (these can usually be bought at a grocery store and should already be dead!).