Regardless of the size they are now they need a bigger tank. Regardless of whether the naso is small or not it needs space to swim which a 55g just doesn't give it which will lead to a stressed out sick fish. Assuming that is a volitan lion then the only fish that actually belongs in that tank are the 2 clowns which will eventually be eaten by the lion fish.. Probably sooner rather than later.
Looking at this for reference on minimum tank sizes:
Volitan Lion - 120g
Saltwater Aquarium Fish for Marine Aquariums: Volitan Lionfish, Colored
Naso (Lipstick) tang - 180g
Saltwater Aquarium Fish for Marine Aquariums: Naso Tang
Emporer Angel - 220g
Saltwater Aquarium Fish for Marine Aquariums: Emperor Angelfish
I would seriously suggest taking them back to the store and trading in now before stuff starts to die.