Fish being slow growers

[Fishy411]

None of my fish have been growing. My pictus's have grown like .5 inch in 5 months, my severum has grown about .5-1 inch in 6 months. My rainbow cichlids has grown and is close to 5" now so i'm not worried about him. I have no idea what it is. I feed flakes, shrimp pellets,lettuce, and catfish pellets. They are in a 55 gallon tank so they cant be stunted. I dont get it. They dont look sick