GloFish are a genetically altered version of the common zebra danio, and while the GloFish group will display far more spectacular colors than their common cousins, care and breeding are entirely the same for both variations. Many new hobbyists are excited to see "pregnant" GloFish swimming in with the rest, but this may actually be a misconception.
GloFish, like regular zebra danios, are sexually dimorphic at adulthood. This means that you can tell the difference between a male and female danio or GloFish by physical appearance. Males are slimmer and often bolder in color, while females have more rounded bodies.
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Because female danios and GloFish display this curved, round belly at adulthood, many new fishkeepers mistakenly think that their adult female fish are pregnant. Some females are rounder than others, with a distinct pot-bellied look, but all will develop this appearance at maturity.
In reality, GloFish are not a live-bearing fish species. Females carry unfertilized eggs in their bellies from the time that they reach sexual maturity, ready to be fertilized by a male of the species. They do not carry live young for a set gestation period as mammals or even some other fish do. The pregnant appearance caused by a belly full of eggs is what leads to the sexual dimorphism between male and female GloFish.
Breeding GloFish is not a difficult task, and the fish will often spawn readily without the fish keeper's knowledge as long as their basic needs are met. To optimize spawning conditions, the adult fish should be offered a high-quality diet consisting of live and frozen food. After a week or two of conditioning, a large (50 percent or more) water change can help induce the fish to breed. Some keepers also have success with dropping the water level to a few inches. When ready to mate, the female will drop her eggs to be fertilized by the male.
If you would like to keep any fry from the resultant mating, the parents must be removed from the tank immediately after they spawn, or they will generally proceed to eat the eggs and any fry that manage to hatch. A breeding tank with a piece of plastic craft mesh covering the bottom can act as a good egg trap, preventing the parents from reaching the eggs as they fall.