Paradise gouramis, also called paradise fish, come from the same family as the Siamese fighting fish or betta fish. Paradise gouramis have temperaments similar to these, the males of the species having territorial disposition. You need to be able to identify males, to avoid bloodshed.
Coloration is probably the quickest way to tell a mature male paradise gourami from a female. In general, males have brighter, more intense orange and blue markings than their drabber female counterparts. Several color variations of this species exist. In all color varieties, males are always the more colorful fish. Gender identification according to coloration works best in mature fish, since immature fish tend to have drabber colors and resemble females of the species.
You can tell a male from a female by the shapes of their finnage. Males have larger, longer fins than their female counterparts. Additionally, the fins on males are more pointed than the fins of female paradise gouramis. When they are sexually mature, this difference becomes more apparent. Males develop long, trailing extensions from the tips of their fins.
You can spot a male by his behavior. Like his close relative the betta, the male paradise fish is more aggressive than the paradise gourami female. A male paradise gourami gets belligerent toward other same-species males and even females, and males of other species of fish. Females get along fine and will not bother each other or other species. Groups of female paradise gouramis can share an aquarium without incident. But if two fish are fighting with each other, they are almost certainly males.
When breeding, the differences between male and female paradise gouramis become more obvious. Males tend to flare their fins out, making the larger size and more pointed shape more obvious. Males will also get more combative with other fish. In fact, they may pick on females who are unwilling to mate. Males construct bubble nests, structures made out of mucus bubbles and plant bits, to house their eggs. Only male paradise gouramis make nests, so if you see a fish making a bubble nest, you know it is a male.