brown and clown are totally different. the one with the spots [ stripes when they are young ] grow about a a foot and a half their first year. largest i've personally seen was 2' long and probably 10" + tall. they can get 3-4 foot.
the brown knife or african brown grows slow and stays smaller. rumored to reach 12". i see mine going to the surface for air all the time [ normal for a brown] but i rarely see him eat. i do give him guppy fry every now and then. he's about 5" and got a little belly on him. he's only grown about 1.5" since i bought him 9 months ago. he is such an different shape, the other fish [cichlids] dont bother with it. they just pretend he isnt there.