I just typed the following message about congos before reading previous replies and finding them already recommended:
How about some congo tetras? They're really nice looking, very active fish. If you were planning on 150 cardinals, maybe get 30 congos? They get reasonably large. If you have plants they might nibble them, so be careful with that. They only seemed to eat my anubias, which was weird (tough plant).
In addition to the congos, african brown knife, african butterfly fish, senegal bichirs (warning: bigger and more aggressive bichirs will successfully hunt the congos), ropefish, synodontis cats, elephantnose fish for some "cool factor." Congos to draw attention, knifefish/butterfly fish/elephantnose to keep it. (warning: jumpers and escape artists listed)
How about some congo tetras? They're really nice looking, very active fish. If you were planning on 150 cardinals, maybe get 30 congos? They get reasonably large. If you have plants they might nibble them, so be careful with that. They only seemed to eat my anubias, which was weird (tough plant).
In addition to the congos, african brown knife, african butterfly fish, senegal bichirs (warning: bigger and more aggressive bichirs will successfully hunt the congos), ropefish, synodontis cats, elephantnose fish for some "cool factor." Congos to draw attention, knifefish/butterfly fish/elephantnose to keep it. (warning: jumpers and escape artists listed)