How Do I Breed My Goldfish in a Tank?

Goldfish are one of the most recognizable of aquarium fish. They have much to recommend them. They're extremely tolerant of water temperature and quality, which makes them very forgiving to the inexperienced fish owner. Additionally, they come in dozens, if not hundreds of varieties, and in myriad colors, including silver, red, gold, blue and black. However, despite the ease of keeping goldfish in an aquarium, they're one of the trickier fish to breed. It's not impossible, though -- you'll just need some patience, and plenty of space for the fry.

Things You'll Need

  • Twenty-nine gallon aquarium
  • Digital aquarium thermometer
  • Aquarium heaters (one per tank, minimum)
  • Spawning mops, hornwort, and/or baby grass (available at pet shops or online)
  • Aquarium divider
  • Methylene blue (available at pet shops or online)
  • Twenty-gallon "long" aquariums for raising fry
  • Sponge filters
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Instructions

    • 1

      Determine the gender of your goldfish. This one of the trickiest parts of breeding goldfish. In mature fish (fish larger than 8 cm), females are larger and more plump, while males are slimmer. Males often have "spawning tubercles," small white bumps on their gill covers and heads.

    • 2

      Set up your breeding tank. It should be big enough to hold fifteen gallons of water per fish. You should install the tank divider halfway down the tank, with dense plants and/or spawning mops. This will protect the eggs from their parents, which will eat the eggs if they can get to them.

    • 3

      Put one female and one or two males into the tank, separated by gender. Then, lower the temperature (no more than two degrees per day) of the tank to 50 degrees Fahrenheit. One way to do this is to put the tank in an unheated room, such as a garage in the winter, and adjust the aquarium heater's thermostat down the two degrees per day.

    • 4

      Raise the temperature, two degrees per day, back up to 70 degrees Fahrenheit. This simulates spring weather, which is a trigger for goldfish to spawn. You should see the female begin to swell with eggs and the spawning tubercles on the male become more prominent.

    • 5

      Assemble a "baby tank" for the fry. Fill a 20-gallon fish tank with six inches of water. Install an aquarium heater and a sponge filter, adjusted so there is minimal water movement. Add a few drops of methylene blue to prevent fungus.

    • 6

      Remove the fish-tank divider. The fish should spawn within several days. In the spawning, the male will chase the female and push on her abdomen.

    • 7

      Remove the spawning mops, plants and/or baby grass and place them in the baby tank when the fish have stopped spawning.

    • 8

      Rinse the eggs in tank water and place them in the baby tank. Remove any dark-colored eggs; they're unfertilized and can foul the water. The eggs should hatch in two to three days; they'll rise to the surface just as they're about to hatch. The water should be at 70 degrees Fahrenheit; raise it two degrees per day if it's colder.

    • 9

      Feed the fry two days after they hatch. They'll eat oatmeal paste or hard-boiled egg yokes. Siphon off any uneaten food after twenty minutes. The food should be finely ground.

    • 10

      Remove the fry to other tanks or cull them as they grow. Unfortunately, goldfish have huge spawns so it's unlikely that you'll have enough space to raise all of the offspring unless you have many tanks or several outdoor ponds at your disposal.