i think this is really dependant on a lot of things, for example tank shape. if you were to house fish in a deeper tank then larger barbs come to mind, also decoration would this tank be planted or bare? besides answer to question... best thing to keep in a large tank is a load of live rock in a huge salt water aqauriam, and watching to see what happens. my fav freshwater is the red tail cat, mostly for its personality... mostly.