on a gt, you tell the difference between male and female by the fins..males fins are longer and pointed, and females are rounded..the gt will get around 8-10'' in aquarium..possibly larger, they are SLOW growers though..ive had mine about a year and its grown a wopping 3 inches if that..
blue acara's get anywhere from 5 to 8 inches in length, and are also very beautiful fish, like the gt..you can tell many differences of the 2 fish. for a blue acara i would recommend a 40 long or 40 breeder at the minimum..55 would be great though :] blue acara are aggressive fish, not as aggressive as the gt, but pretty close..they'll eat just about anything you drop in there, where in contrast, my gt is a very picky eater.