Thats a great size tank! Like everyone said the possibilities r endless, but I would probably do smth like this:
for the bottom BGK, fire eel, polys (and maybe even elephant nose fish, it might work with the bgk in a tank that big)
for the middle Dats, clown knife, maybe some vieja, flagtail and a school of pink tail chalceus ( ppl usually keep one but with a tank that big u can have a school of 6!!!)
for the top a couple of aros and maybe one gar.
for the bottom BGK, fire eel, polys (and maybe even elephant nose fish, it might work with the bgk in a tank that big)
for the middle Dats, clown knife, maybe some vieja, flagtail and a school of pink tail chalceus ( ppl usually keep one but with a tank that big u can have a school of 6!!!)
for the top a couple of aros and maybe one gar.