how much help was my answer for you? that's why it was pointless. I gave you the only possible answer with the info given. now if you had asked what fish would be happy in your 48"x6"x12" we would know that you wouldn't want any fish that get over 4" with 3" being even better. how would we know if a fish gets too big for a tank without knowing the size of the tank?
and you certainly should point it out if I ask a pointless question, how else would I learn?
just use common sense. if the tank is only 12" wide then you probably shouldn't do fish that exceed 12" or can't turn easily. only exceptions could be stuff like eels or bichirs.