Female festae would work, since they don't get really big and are slow growing. The males are much larger though. The tank is too small for an oscar, since they can get longer than 13 inches. A GT would also work great. They get 8-10 inches max, and a little smaller for females. A gold saum and silver saum are the two types of the false 'green terror' Aequeidens rivulatus, the true green terror, is very rare compared to the saums, and is a bit more aggressive. You can get one from Jeff Rapps though. Another option is a salvini pair. They are very beautiful and only around 7-8 inches for males. Texas may work, although a 12 inch fish is too big for a 48x13 tank IMO.