I have 3 fire eel and a tire track eel for 8 years I have salt in my water not a brackish tank but it keeps the fire eels safe and healthy no fungus or sore on my Eels. ever
I have kept a lightly brackish tank for over 10 yrs, and I've always had a fire eel. The first died in a moving accident after 7yrs. They seem to do fine in the 1.002-1.006 range. see my tank link for more on mine. . .