How to Determine Whether a Fish Is Male or Female?

Fish are typically not sexually dimorphic; there usually isn't a very distinct physical difference in appearance between the males and females. Some species, though, have some telltale signs that vary among the genders such as color or size. These are particularly prevalent in livebearing fish or fish where courtship has an important role. While there is no clear cut rule that is applicable to all fish, there are general guidelines you can follow to determine the gender of any species of fish.

Things You'll Need

  • Aquarium with fish
  • Aquarium fish net
  • 3 buckets of aquarium water
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Instructions

    • 1

      Determine whether your fish is a live bearing fish. Live bearing species include fish such as mollies, guppies, swordtails and wags. If you are uncertain of the species, check with your local pet store for identification.

    • 2

      Carefully scoop a fish out of the aquarium using the aquarium fish net for close analysis. If your fish is a live bearing species, moisten your hands with aquarium water. Gently hold the fish as still as possible without squeezing too tightly; check the underside of the fish. Look for a modified anal fin beginning midway on the underside of its belly trailing back to the vent near its tail. The fish is likely male if you can locate this fin since it's absent in females, and this fin is the biggest key gender difference in live bearing fish.

    • 3

      Use the net to scoop multiple fish of the same species for analysis if the species is not live bearing; comparison is the best bet if no specific guide is available. Select and hold one fish still at a time to locate the vent near the tail.

      Note the size of the vent. Check the vent size of the other fish for comparison. Females will generally have distinctly larger vents than males in most species of fish.

    • 4

      Put determined males into bucket one, females into bucket two and undetermined ones into bucket three. Recheck the fish in the undetermined bucket once you are finished with the ones from the aquarium. Do this no more than two times to avoid putting too much stress on the fish. The fish should also be out of the water a maximum of thirty seconds each time for the same reason.

    • 5

      Place all the fish back in the aquarium. You may also opt to use your sorted fish for breeding or other identification purposes.