The tank is to small for sds and pirhanas i'm afriad, how about some dwarf cichlids like apistogramas,bolivian rams,anomalachromis thomasi,laetacara curviceps? a couple of pairs would be entertaining and you could keep quite a bit of your stock by the looks of it Also a pair of small-medium cichlids could work, I would say that you could just about scrude a pair of thorichthys species like ellioti of firemout if it is colour you are looking for. Maybe 2-4 Angelfish as your tank looks quite tall so will be ample swimming room for them until they get to full adult size.
What about adding some wood in there? then you could scape your plants around the wood and the rocks in two areas so it would look quite simple and would look really nice also some floating plants like amazon frogbit of duck weed.