are you planing on geting a bigger tank soon bc if so then you can get some cool baby monsters and grow them out. if not then we will haft to think of some other fish.
i would go with one pair of 6" cichlids like black or blue acara or salvini
i have a pair of black acara aka port cichlids and they are happy in there
or some smaller bichir or 2 it would take a couple years atleast to out grow it
collect fish from a local water way as well as plants snails crayfish insect larvae ect
just a few ideas