Zebrafish (Danio rerio) are small freshwater fish often used in scientific experiments, because they are vertebrates and lay large quantities of eggs. Zebrafish can also be kept as pets, and make a great addition to an aquarium. They measure approximately 1 to 1 1/2 inches and are available at most pet stores. If you add a zebrafish to your aquarium, you may want to determine whether your fish is a male or female.
Look for golden tones on the zebrafish. Male zebrafish tend to be pinkish and yellowish, while female zebrafish tend to have blue and white colorings.
Examine the body shape of your fish. Male zebrafish tend to be slender, while female zebrafish appear more rounded. If you have just purchased your fish, however, keep in mind that they may have been underfed in the pet store.
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Wait to see if a fish swells. This indicates that the zebrafish is female.
Observe your fish in the early morning. Male zebrafish tend to chase female zebrafish before they breed.