Male Jack Dempsey fish are significantly larger than the females and boast slightly longer fins. The brilliant pattern of metallic spots on males is also more colorful than the pattern found on females. The tail, anal and dorsal fins of males are pointed, but the fins of female fish are slightly rounded.
Although it may be difficult to determine the sex of juvenile Jack Dempsey fish, the sex of adult fish is easier to identify. While tolerant of other fish species, males do not tolerate the presence of other males, so if two fish acting aggressively towards each other, both fish are probably male.