fire eels are freshwater, i would add more then one eel as they can be vary shy, but they also tame pretty easy mine would beg for food and ate prawns and scalops from my hand, make sure you have a good top i thought i did but mine still jumped out
, it took mine two years to grow to 16" and they can get three feet long but it takes a long time!, they will probably eat shrimp and crayfish may pinch them they are vary soft bodyed and get fungus easy, if they have a place to hide they dont normaly come out except for food, mine would come out and swim to top of the tank when he saw me comeing, they are great fish to have and even better when they get bigger, but that size tank is a bit of waist with just one or more fire eels, mabie try some bichirs, aros, ropefish, dats or something as well?