How To Tell The Sex Of GloFish

GloFish are genetically modified fish specifically engineered to glow bright neon colors under light. Zebra danio GloFish were the first GloFish species to be invented. Scientists injected an embryo with fluorescence protein genes from other marine animals. Offspring of this altered zebra fish carried this gene, making subsequent generations glow. Originally, this was done to create an environmental pollution indicator.

In 2003, GloFish became popular as pets thanks to the efforts of the GloFish company, which trademarked the name. Other fish species genetically engineered to glow include barbs, tetras, and bettas. Therefore, determining the sex of your GloFish is similar to determining the sex of zebra danios and neon tetras.

At the time of writing, GloFish LLC states that "intentionally breeding or selling GloFish is strictly prohibited." This is primarily due to licensing laws.

GloFish owns the trademark to the genetically modified fish, and as a result, they are the only company that can legally sell them. However, if your GloFish happens to procreate without your interference, that's not necessarily illegal. You just would not be able to sell them for monetary gain. Doing so could result in a fine. Therefore, you will want to limit your GloFish's breeding abilities, so you'll need to know how many males and females you have in one tank.

Like other community fish, such as guppies and tetras, GloFish should be kept in tanks of six or more. Determining the sex of your GloFish isn't easy, but it's similar to the same method you'd use to determine the sex of a tetra or a zebra danio fish. However, the additional neon orange, yellow, and green colors can make determining whether your GloFish is male or female more challenging.

Determining whether tetra GloFish are male or female involves looking closely at the color of the stripes extending across the body. GloFish are not injected with colors or dyes; the glow is a result of genetic modification. When placed under white or ultraviolet light, they absorb light and remit it, appearing to glow. Thanks to the fluorescent colors, the characteristic tetra blue stripe can be harder to see, so use a bright light. Both male and female fish have this blue stripe.

Now, look below the blue line — there should be a second stripe. If it is gold, the tetra GloFish is a male. If the stripe is a sparkling silver, the fish is a female. Also, male GloFish are slightly smaller and thinner than females, but to see the difference in size, you'd need a comparison of both male and female in one tank. If you have only one gender, size differences won't be obvious.

There are currently six different GloFish species, but the first GloFish species created was the zebra danio GloFish. Most GloFish sold as aquarium pets are either tetra GloFish or danio GloFish. Determining the sex of danio GloFish isn't easy. Use the same strategy as sexing zebra danio: compare the size of the fish.

Female danios are usually wider than the males, who have sleeker bodies. Like tetras, female danios might look pregnant if they are carrying eggs, as their belly will bulge. However, neither danio nor GloFish give birth to live fry, so they cannot technically be pregnant. Female danio GloFish will appear wider before laying eggs.

Per Aquarium Co-Op, GloFish should be kept in freshwater tanks that are between 20 and 40 gallons in size at the minimum. Ideally, GloFish should be kept in temperatures between 72 and 82 degrees Fahrenheit. PetCo recommends adding a heater to the tank to stabilize the water temperature. Any changes greater than two degrees can stress the fish. 

GloFish are omnivores and eat both plants and animals, like brine shrimp.