A shoal (5) of Gourami's, with 5-7 Cory cats as a cleanup crew!
Or tiger barbs and cory cats?
BTW those are the dimensions of my '50' gallon as well, although mine is actually closer to 11.5" usable width...The 55's are 13" (12.5" usable) wide...
I currently have a blue gourami, a younger BGK, and (hopefully) a breeding pair of bichirs, one Senegal and one albino short body...Pulled these 2 from my 90gallon as my sen was getting aggressive towards a growing palmas palmas...