the 'back fin' i assume he means the tail fin, or the caudal fin. the fin on the fishes back is the dorsal fin. on my blue acara both of these fins have an orange tint to them, but not like the green terror...the green terrors fins are vivid orange, where as the acara's are very light. they are in the same family, they will look similar. there is no way to know for sure if ANY fish is 100% the species it was labeled or is supposed to be, and on that note the fish that the OP posted a picture of, has all the features of a blue acara, and yes...it will have some resemblance to a green terror but it is in fact not the Aequidens Rivulatus, but an Aequidens Pulcher. mine chases around everything in the tank as well, and he also lip locks with my green terror, he is aggressive, and is more aggressive than my jack dempsey..thats why he lives in a tank with 3 severums now, and he doesn't have any problem with them