Angels. Severums are too big for a 55 gal. The rule i go by is...
The width of the tank should be at least 2 times the length of the fish (so the fish can easily turn), and the length should be at least 3 times the length of the fish (so the fish can swim without immediately hitting the end)
The width of the tank should be at least 2 times the length of the fish (so the fish can easily turn), and the length should be at least 3 times the length of the fish (so the fish can swim without immediately hitting the end)