Although spotted green -- or green spotted -- puffers (Tetraodon nigroviridis) are often sold as freshwater fish, they can also thrive in brackish environments. These fish can be hard to maintain, so best suit more advanced fishkeepers. While you may want to know the sex of the fish you're taking home, this can be hard to determine.
About Spotted Green Puffers
Spotted green puffers are native to Thailand, India, the Philippines, Myanmar, Cambodia, Sri Lanka, Indonesia, Malaysia and Vietnam, where they live in freshwater rivers, streams and lakes as well as in brackish coastal waters. They're relatively large fish that can reach around 6 1/2 inches in length. They can be kept in either fresh or brackish water in a home aquarium, but adults of the species tend to thrive in a slightly saline environment. They like tanks with plenty of plants and rocks as cover, but also lots of space to swim around in. A 50 to 60 gallon aquarium is ideal.
Dude Looks Like a Lady
It's extremely hard to tell the difference between male and female spotted green puffers as they show no external sexual dimorphism, which means that both sexes look exactly the same. Males are neither larger nor smaller than females. While coloration can vary slightly between fish, their sex has no influence on their coloring. These puffers tend to be a rich green on the tops of their bodies with darker spots and their bellies are white or off-white. Juveniles of both sexes may have duller coloring than adults.
One way to identify a female spotted green puffer is that she can lay eggs. If you happen to get both a male and a female of the species and they mate, the one who produces the eggs is the female. She will lay roughly 200 eggs on a smooth, flat and protected surface, such as a rock. Although commercial captive breeding attempts haven't been successful, hobbyists tend to have more luck. These puffers are more likely to breed in brackish water.
If your spotted green puffers lay eggs, you will notice one of the fish hanging out around the laying site. This will be the male, who has been noted to exhibit guarding behavior. He will only guard the eggs until they hatch, which usually takes around one week. In the wild, once the fry have hatched they'd be expected to look after themselves. However, in captivity they should be fed microworms and young brine shrimp.