Turn the heat up to 84F and add aquarium salt (2 tablepoons per 5 gallons). Get an over sized filter (an Emperor 400 works great in a 55) for a nice strong current. Get him a hiding place so he can escape the current if he wants to. Lower the lights and keep them low (on a 55 you should normally use 2 light bulbs - take one out on the side where he hides). Feed 3 to 4 very small, diverse meals daily with one day per week of no food. While they are small (under 12") they are fragile when it comes to disease. No feeders! Unless you grow them yourself or use earthworms. Feed him at night as much as possible; either with no light on, or a blue LED "moonlight". Feed on opposite side of tank from hiding place so he has to work for it. Try Hikari Carnivore pellets (you may have to cut it into quarters until he is big enough to swallow them whole). Bits of fresh tuna and shrimp are good too. Tuna is really high in protien and they love shrimp. Many CKs also like small Hikari algae wafers.
The heat and small meals will boost their metabolism. The current will exercise them. The salt will promote slime coat and healthy skin. Salt also helps heal scratches on their skin which can effect appetite.
