Whether your goldfish reside in a pond or an aquarium, breeding follows the same process. The right combination of temperature, lighting, differing sexes and a watchful eye can, in the span of four to five months, fill your tank with healthy little fry.
According to records, goldfish have been kept as pets as far back as A.D. 300.
If the Water's Warm
Most goldfish will happily breed in captivity, but need a cue from their environment to do so. These cold water fish only get that twinkle in their eye if the water reaches a certain temperature. This is their trigger to go forth and procreate. If their tank or pond is typically unheated, the regular winter cycle of shorter days and colder nights is often enough to begin the process. If their tank is heated, lower the temperature to 58 degrees Fahrenheit. At the same time, reduce the amount of lighting the tank or pond receives to only five to seven hours a day.
After you've given the fish this pseudo-winter period for two to three months, gradually increase the lighting and temperature. Over the course of 30 days convince the goldfish that spring is in the air, until the habitat is receiving 12 hours of light per day, and is a balmy 70 degrees. This should trigger their instinct to spawn.
Nothing Says Love Like Tubercles
"Fancy" goldfish isn't just a name; during mating season, male goldfish get a little gussied up and grow tubercles on the sides of their gill covers and pectoral fins. These little white spots are sometimes mistaken for ich, a common fish disease, however unlike the random full-body spread of ich, tubercles are patterned neatly along the fish.
Tubercle growth is boosted by the increase in testosterone in the fish, and no goldfish sports the same tubercle pattern. While the tubercles don't appear to have any clear effect upon mating, they do allow the savvy fish owner to differentiate sexes.
During this time of tubercle growth on the male, the female goldfish will undergo a few bodily changes. As she becomes full of roe, or eggs, her midsection begins to swell. The closer she is to scattering her eggs, the more this swelling will increase; enough to press upon her anal area, causing it to bulge slightly.
Unlike French angelfish or clown fish, goldfish aren't the "until-death-do-us-part" types. Not only will males not stay with their mate, but they literally will chase any tail they can find in the habitat. One male can breed with several females in a single pond or tank.
Furthering their short-term relationship philosophy, goldfish don't make very good parents either. After fertilization, you will want to remove the adult fish -- or all of the fry -- from the habitat, as even parent goldfish will eat their own eggs.
The Down and Dirty
The first indications of spawning behavior are just as easy to spot in an aquarium as they are on any college campus. The male goldfish will begin to chase as many females as there are in the tank, nudging their stomachs. This nudging, which could be mistaken for bullying, is an attempt to get the female goldfish to release her eggs.
When momma goldfish does begin to lay her eggs, she'll do so in batches. She'll release them in any live or fake plants in the habitat, and the first batches will be the most fertile. Eggs that turn an opaque white are infertile, and should be removed from the tank.
Goldfish eggs will stick to the first things they touch, and often clump up. This helps the male goldfish, who will immediately fertilize the eggs by spraying them with his milt, a combination of sperm and seminal fluid. The whole process takes about three to four hours.
Each female goldfish can lay up to several thousand eggs.
After you've removed the eggs to a hatching tank, you can keep an eye on their growth. If you see a small black spot in the center of the egg, congratulations. In about seven days, you'll be welcoming tiny little goldfish fry into the world.