I've probably got the most overstocked tank in this thread:
Oscar 11"
Amphilophus zaliosum 5"
A. xiloaensis 7"
Herichthys carpinis 7"
ex-"Cichlasoma" salvini 6"
"Aequidens" sp. Gold saum 6"
Parachromis friedricksthalii 6"
P. loisellei 6"
P. motaguense 4"
All in a 75 gallon!!! Everyone gets along except for a little bit of chasing around, but these are all particular aggressive cichlids. Obviously this a growout tank, it would crazy to keep all of these fish full grown in a 75. Oh and it's also worth noting, whenever I add a new fish, it's toast.