i love that tank, not overstocked, great selection in fish, very natural looking, with nice plants and lighting...but becaquse i love catfish, i think a bullhead would fit i nicely in that tank. haha as for the OP generally fish do best with fish that they would natively be found with. This is a good rule of thumb, so many of the sunfish species, or bullheads, crappie, perch...yo ge tthe idea. just try to keep in mind sizes so no one gets eaten. but Mr. Catfish's tank is an awesome example of a healthy tank with fish getting along quite well.