Could do a large community of small-ish growing CA cichlids. I wouldn't do fish that grow over the 8" mark because there is only 3' in any direction for potential victims to run, but with this many gallons you could do an amazing setup of all different amatitlania/cryptoheros, smaller hericthys, etc. One of my "dream setups".
I would pick, say, 4 species and do groups of about 10 each. Some good species would be:
P. panamensis, any amatitlania fish such as sajica, thoricthys maculippinnis, hericthys tamasopoensis, Centrarchus, P. nebuliferus. Plenty more CA's in this size range.
Then I would also do a large shoal of ameca splendens. Very nice livebearer from mexico that is more boisterous than swordtails, etc. and hold up to CA aggression well. They also give birth to very large fry which would have a better chance of surviving, some would get eaten but you could have a steady population.
Alternatively, you could do something just like this, but with all SA cichlids. That has been done much more than this, and would be easier to get to work. The CA cichlids are nastier to each other, and the setup I mention above probably would require a bit of work to get the mix right.
