Dlight, I'm not sure of your references but the two sources I have both label the clown as getting bigger then a niger. Look it up. No, the clown probably won't get 20" but book says 19.9" meaning it may get anywhere from 14" to 19" depending on its genetics=) Male vs. female,etc,etc. And the fact that I stated it would get "ALMOST 20" which would make the statement correct. Just as there are shorter and taller people. But references including "Marine Fishes" by Michael (in front of me now) say the clown gets larger than the niger. Clown=19.9" and Niger=19.7". Not alot of difference there but I think the niger's long tail is included. I've personally seen a clown that was probably 15" or so and have never seen a niger that large (not that it does'nt exist).
Anyways, not debating with ya'....just clarifying my info and the point of a clown getting larger than a niger. And of course a whole lot meaner in that small of a tank. You are much safer putting in the smaller more "layed back" niger.