How to Breed Goldfish

Goldfish breeding is usually fairly difficult in an indoor tank but is much easier and happens spontaneously in large tanks or ponds. Alternatively, installing a breeding tank with a good amount of water and proper set-up can give good results.

A goldfish usually needs to be 1 year old before reaching full sexual maturity. Between May-July, the male develops breeding tubercles (sometimes called pimples or breeding stars) at the back of the head and sometimes on the pectoral fins. Females tend to be deeper-bodied than males. During spawning, females develop a rounded and soft abdomen.

If the conditions are right – healthy fish, clean water, good quality food, temperatures around 20° C-23° C (68°F-73°F), the male will start chasing the females from dawn until early afternoon. The female will release several bursts of gooey eggs (between 500 and a few thousand) – which adhere to plants, vegetation and decoration.

In the main aquarium, however, the eggs and fry tend to be eaten by the goldfish so installing a separate breeding tank will give much better results. To install a breeding tank, you will need:

• A tank of a 75 to 100 liters (approximately 20-25 US gallons)
• An aquarium heater and a tank thermometer
• Add numerous aquatic herbs of the Elodea type and live plants with fine leaves: the eggs will stick to the leaves then develop and hatch. Alternatively, install a spawning mop to catch and protect the eggs (click here to learn how to easily DIY a spawning mop)
• Avoid any strong current in the breeding tank. Install a sponge filter (rather than a power filter) to avoid sucking the fry.
• Temperature of the tank: 20°C/68°F to 23°C/73°F.

Place one male for two females in the afternoon before spawning, which usually takes place in the early morning. Remove the fish as soon as spawning is finished so that they do not start to eat the eggs. The eggs are laid in bursts, usually in the early morning and immediately fertilized by the male. The viable eggs are the most transparent, the others, whitish, will not hatch and will probably be subject to fungi: remove them if possible so as not to risk contaminating the fertile eggs.

Goldfish fry at 3 weeks – Photo : Bristol Aquarists Society.

The eggs hatch around 3 – 5 days later. The fry should not be fed for the first 48 hours because they have their own reserve in the form of a small bag and they stay inactive for a couple of days. When they start swimming around, start feeding them with special fry food, crumbled egg yolk and artemia nauplies. After 2 to 3 weeks, switch to finely crumbled flakes. Feed the fry often but in reasonable quantity and regularly check the water for ammonia, nitrite and nitrate levels (high risk of mortality or deformities otherwise).

The water changes shouldn’t be too sudden – changing a small amount of water at a time but often. Use a dechlorinator (water conditioner) to eliminate chlorine and chloramines from tap water.

Don’t add any baby fish to the main tank until they are big enough. The current created by the main tank filter may be too strong for them. And if the small fish can still fit in an adult goldfish mouth, they will become a meal!

Did you know?
Goldfish breed in spring, when temperatures start to get warmer. To simulate this temperature change in the fish tank, some fishkeepers progressively lower the temperature of the tank to approximately 15°C (59°F). Then, in order to induce breeding, they raise the temperature of the water by 2°C (3°F) per day, until it is between 20°C/68°F to 23°C/73°F.  In a similar way, in the few weeks before the first spawning, it is possible to gradually increase the amount of light available in the tank, for example from 8 hours to 11 hours of light per day. To help females develop eggs, you may also feed your fish with more proteins (frozen artemias, brine shrimps, bloodworms and daphnias) – if you do so, keep an eye on your water parameters (ammonia, nitrites) and increase partial water changes to avoid any water pollution.

Important: Before breeding your goldfish, consider the space available in your tank or pond. Do you have enough space for the new fish in addition to your existing goldfish?

Note: Avoid using activated carbon (charcoal) in your filter if you want to breed your goldfish. Activated carbon can interfere with the hormones goldfish produce to trigger breeding.