I don't think any traditional "wet pet" is going to work in that footprint but you still have some good options for species that are much more interesting then run of the mill community fish. A few apistogramma could work in there. Maybe a pair of rainbow cichlids but I'm not sure of the footprint of that hex. Perhaps a pair of a smaller species of jewel cichlids but breeding activity could end up with one killed if it can't escape. Kribensis also stay smaller and could be cool in there.