These two fish are often confused in the aquarium trade due to their extremely similar physical characteristics as juveniles. Their common names are not often confused, and they are quite distinguishable as adults, but they still are labeled incorrectly. In the following, I'll give you a few pointers on how to tell these two fish apart.
The first issue I want to address is the true identity of the Green Terror. The original green terror is the fish Aequidens rivulatus originating from northern Ecuador. It was first described in the 1800's but since has been confused with a few other species. “Aequidens” sp. goldsaum, and “Aequidens” sp. silbersaum. The goldsaum originates from Ecuador, and the silbersaum from Peru. Some believe that these fish are all separate species, and some believe that they are all one species with different physical characteristics defined by habitat and diet. Officially, there is only one fish described and that is A. rivulatus which is why the other “Aequidens” are highlighted in quotations. Although there is confusion among the identity of the True Green Terror, one thing is known. The fish sold in almost all pet stores is “Aequidens” sp. goldsaum, which is commonly referred to as the Green Terror. Confused? Join the club.
This is a large growing cichlid. Large specimens have reached 12 inches though 10 inches is more common for an adult male, and females slightly smaller. Green terrors have a long bodied profile, but as males mature they often gain height and bulk. Males are capable of developing large nuchal humps similar to A. citrinellus. Their caudal and anal fins are lined with an orange-red band, and their face is marked with worm-like iridescent blue markings. The body color varies from yellowish-gold color to a greenish-blue hue. Truly they are a beautiful species that I recommend for any large cichlid seeker.
Blue Acara's are a medium sized cichlid. Large males have been known to reach over 8 inches in length, but 6-7 is more common, females being slightly smaller. Blue acara's have a long bodied profile and are known for their long dorsal and anal fins. The dorsal and anal fins lengthen toward the caudal fin, giving the fish a unique shape. While males are bulkier and have a heavier forehead than females, they typically do not develop the large nuchal hump like the Green Terror. The slope of their forehead is not as steep as the Green Terror. Their color is typically a gray based body color horizontally lined by a gold, greenish, blue color, often broken up by black vertical bands. They also have iridescent blue facial markings accompanied by a vertical black eye mark. Their caudal and dorsal fins are often lined with an orange-red band like the terror, however, the Terror's band is much thicker than the Acara's.
Below are a few side-by-side pics of Green Terrors (left) and Blue Acaras (right). I've circled and pointed out a few of the characteristics that will hopefully make differentiating these two fish a bit easier.
The blue circle on each fish is showing the difference in head shape. When these fish are juvies, many times this is the only way to tell the difference between the two. The Green Terror has a steep sloped forehead, and the Blue Acara has a more feminine, slow sloped forehead. The red circle is showing the difference between the body profiles. The body height of the Green Terror is higher than that of the slender Blue Acara. The white circle is the most reliable way to tell Terrors from Acaras. Most Acaras do have an orange/red caudal band like the Terrors, but almost never as thick as a Terror's caudal band.
Note the arrow pointing to the Green Terror's nuchal hump. You'll never seen an Acara grow one of these.
Note the arrow at the bottom of the Green Terror pointing out the body coloration. The color is a vibrant greenish-blue color that a Blue Acara won't display.
Note the arrow pointing to the eye mark on the Blue Acara. I have seen some Green Terrors with this mark, but not many. Mostly, it is a characteristic of the Blue Acara.
Note the arrow pointing to the vertical bars on the Blue Acara. Most Acara's have atleast faded vertical bars as a part of their body coloration. Green Terrors typically don't show vertical black bars unless they are stressed.
This article is meant to be used as a guideline. The information provided here was derived mostly from my own experience or from information I've learned from others. Again, the pictures here are pretty cut and dry, but in the pet store it might be a bit more difficult to identify them. It takes practice and a bit of experience to really be knowledgeable in identification.
Special thanks to: Ksane (Figure A1), bikim420_2005 (Figure A2), Greenterra (Figure B1), Cichlid2006 (Figure B2) of