Single fish tanks tend to lend themselves to either the spectacular...of my persoanal favorite...the interactive. That said I'd probubly go with either a tigrinus or a single orino and work on training it.
are you serious
a betta for a 180g tank
everytime you want to see the fish you'd have to have a search party look for him before you find him
if i had a spare 180g tank for a single fish i would think about an electric catfish