I was in a LFS in Maryland last week, and in one of their many crowded 15 gallon tanks, to my surprise I saw a baby (6-7") pima.
I am of the opinion that you should need a permit for arpaima, arowana, pacu, red tail cats, iridescent sharks... any fish that grows larger than about 18".
I think the rampant abuse and neglect of these fish is a real problem, and unless regulations are placed on purchasing them, it won't be addressed. The one-in-a-million customer who can actually house a pima, if they really want one, will be willing to apply for a permit. Very small price to pay for saving the lives of all the poor fish which are sold to inadequate homes.
Hell, I would be willing to file a permit for a neon tetra if they wanted me to. I personally think the animals' welfare should come first, and filling out a little paperwork, paying a small fee, waiting a short amount of time, are all insignificant inconveniences when weighed against the amount of fish that are neglected and abused daily.
I agree with you and its a very admirable thought. The neon tetra example would collapse the aquatic pet trade though which in turn would have its own ramifications with people that depend on those jobs.