Opaline gouramis are a color variation of Trichopodus trichopterus. They're also known as three-spot, blue or marbled gouramis. These pale blue, pearlescent fish are labyrinth fish, so-called because they have labyrinth organs that allow them to breathe surface air. Physical sex differences are subtle, behavioral ones not so much.
Size and Color
Wild male gouramis are more brightly colored than females, so are males of the original strains available in the pet trade. Not so with newer strains such as the opaline gourami. These guys are not bred so much for their color as for the iridescent sheen that covers their scales. Males may be more vibrantly colored, but the difference is subtle and you'll probably notice it only if you already know which of your fish are male and which are female. Males are slightly larger than females, but size can be deceptive because of age and individual nutrient intake.
The dorsal fin does not deceive. The dorsal fin is the one on top of your fish's back. The tip of the male's dorsal fin comes to a point. The female's dorsal fin ends in a more rounded curve.
If you're still not sure which fish is which, check out the anal fin. This appendage is the fin that runs the length of your gourami's belly. Males' are more pointed, females' are rounder. Females also have a slightly wider space between their anal fins and tails to accommodate their more pronounced, egg-bearing cloacas.
There's no mistaking male opaline gouramis and females according to behavior. Males are not only aggressively territorial, they're the ones filled with parental sentiment and nurturing feelings toward opaline gourami eggs. If you happen to have a male and female, and conditions are right for breeding, the male will build a bubble nest near the surface out of spit. Once he manages to induce the female to lay her eggs in this nest, he'll chase her away and lovingly tend the eggs until they hatch. Mom will have to move out once the chasing begins, since the stress of new gourami parenthood -- that is, the constant attacks -- can prove fatal.