50 gallon fish tank?

I'd find bichirs would do fine in your tank. They're a prehistoric predatory fish with teeth but are one of the more peaceful predatory fish you can get. I think only maybe one or two maximum Senegal bichirs would fit in your tank. And good thing you turned down oscars since your tank is in fact a little to small for a single solitary oscar, plus they are aggressive and really limit your options even in a large tank.

You could do a large school or tetras,

Or a smaller school of rummy nose tetras, their beauties!

You could do african Cichlids

South American cichlids that don't get to large for example convicts,

You could do goldfish

So many choices! If it were my tank I would go with the Africans:)

Good luck, hope I helped!

Small freshwater shark neon tetras angelfish pleco and shrimps