Tigrinus said:I wouldnt go with aros,,,they get too big. Aros need space to swim around constantly, unlike Tigrinus. It is unfair to have a 2.5-3ft fish swimming in 6ftx2ft ovals. You mentioned a 300g so an aro might be ok if you upgrade??
Black aros get just long as silvers,,,they are just not as husky.
It takes a while to get aros to hit 3 ft. At 1.5 - 2 ft , it would be ok to have a width of 24 inches as they have slender bodies. Of course wider is better. Same with the stingrays that you suggested earlier. They would be ok in there for a while. Personally , i would worry about keeping the stingray alive first before thinking about the 20 inch diameter disc that is common with most freshwater rays.