How about a colony of convict cichlids? If I’m correct then the older batches of fry help in raising the younger fry which would be quite interesting in watching.
Never had convicts but always wanted to try them. I don't, however, have anywhere for the fry and equally not sure I want to dedicate the 6 Foot to a colony. I'd also worry about them not using all of the tanks height?
I would definitely consider a pair if I went down the CA cichlid route though.