I don't think it is all that incredible that these fish survive for maybe 4 years.A breeding pair of jags,6 Jack Dempseys, several FHs convicts and Firemouths plus catfish for 4 years in a 120?That's incredible.
Something is definitely kept down. And I doubt most of these fish come anywhere near achieving their potential development. A severe lack of space, add to that frank's idea of filling the tank with excessive decor and essentially taking even more space away ---there could be a status quo and nobody with the opportunity or position to 'rock the boat'.
Add to that almost certain inferior water quality. That in itself sometimes makes some fish less healthy and less aggressive (if all these fish achieve their potential it's over 10 lbs of fish; no amount of water change is going to compensate for that in a 120 gal.)
4 years, even male convict should expect to be something like this:http://i1199.photobucket.com/albums/aa480/bercom/6x13.jpg But I would bet most of these fish remain small and undeveloped.