How to Tell the Sex of GloFish

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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.

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GloFish are genetically modified fish specifically engineered to glow bright neon colors under light.
Image Credit: Mark Wilson/Getty Images News/GettyImages

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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.

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Breeding GloFish isn't illegal, but selling or giving them away is illegal. Also referred to as "glow fish," GloFish are a brand of fish exclusively licensed by GloFish LLC. Only the GloFish company can sell this genetically modified aquarium pet fish. 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.

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Is a tetra GloFish male or female?

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.

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Determining whether tetra GloFish are male or female involves looking closely at the color of the stripes extending across the body. 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.

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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.

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Is a danio GloFish male or female?

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.

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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.

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Keeping Your GloFish

GloFish are freshwater fish and should be kept in tanks of 5 gallons or larger. 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.

GloFish are omnivores and eat both plants and animals, like brine shrimp. To maintain their characteristic colors, tetra GloFish should be kept in temperatures between 68 and 78 degrees Fahrenheit (some sources advise as high as 82 degrees, but that is high for tetra). Water pH can range from 6.5 to 8.

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