exactly fire eels are mastacembelus and mastacembelus dont like each other especialy if its there own kind, ive never had a problem with antisocial macrognathus but i guess its just the personality of the fish.not only the larger, ive found out of personal experience that they only dont get along with there own spp (like 2 fire eels or 2 zigzag eels), i have mixed right now a ZigZag, a Yellow tailled, a baby fire eel, and a baby ellipsifer (from tanganyika), all mixed in a 35g ATM, id say the 20 Long should be fine, but definitely in the 55.