Fish which are termed “exotic” usually have their genes suppressed by the dominant ones and because of that, exotic fish are only kept and interbreed among their own same variety, in order to retain the desired features and characteristics. Although it is very hard to draw a borderline between what you would exactly termed the fish as being exotic or otherwise, eventually that will depend on your requirements and expectations including the context, in which you use to describe it.
For me, I would accept two different varieties of the high-grade fish species and one of them is the Super Red Arowana, which can be quite difficult to find and fetch a very commanding price tag. The Super Red Arowana is indeed a beauty to behold. The fish is considered an endangered variety and due to its popularity, wild species has been captured from its natural habitat until it is almost close to extinction. The variety, which you can see in home aquarium nowadays, has been crossbred to produce different dark red to light tones and they look spectacular against a dark color background.
The other fine example of exotic tropical fish is the Albino Red Turquoise Discus, which comes with its red eyes and display a very brilliant orange to strong red color mix. The strain, which I’ve seen before, has high fin and with its round and thick body, I’m not surprised that the fish is highly sought after among discus owners.