A long tank eh? No arowana for you then i guess. Your best bet would be a community full of moderately sized cichlids (1-2' long). Then you could add in some other cool fish, I'm sure a shoal of clown loaches would LOVE swimming around a 12' tank!
Since this is in the set up and filtrations, imma say.....a sump and a fx5. as for the stocking of fish...I'd go a school of Oscars and a couple of retic rays.