Breeding pair will do fine in a 55 if it's a 48x18 footprint.
Other alternatives, depending on what you can get a hold of (I'll stick to common) are Green/Gold/Rotkiel Severum, Firemouth, Salvini, Blue Acara, and Jack Dempsey's (if you decide you want a fish the same size as GT's)
Other alternatives, depending on what you can get a hold of (I'll stick to common) are Green/Gold/Rotkiel Severum, Firemouth, Salvini, Blue Acara, and Jack Dempsey's (if you decide you want a fish the same size as GT's)