To provide an ideal aquarium environment for the fish to spawn, it is best that the water should be crystal clear and clean, fresh from water changes and the tank bottom should be bare without sands or gravel. A recommended breeding tank should have presence of some live aquarium plants, which you can purchase in bundles from pet stores and then introduced to the tank just for the purpose of spawning.
Once everything is ready, you can start by placing at least one to two mature male goldfish together with a female in the breeding tank. Sometimes, selection is not necessarily needed because it can be quite difficult to find out the gender of the fish and usually placing all them together in the tank and let natural process taking place is good enough as it will ensure there is at least one breeding pair. The water temperature must be maintained cooler than normal aquarium setup for goldfish which is at least 2 – 3 degree Celsius lower than what you normally control in your tank. Usually based on my own experience, 16-degree Celsius should be sufficient and ideal to induce spawning to take place.
During the intended reproduction period, avoid heavy feeding and instead introduce some live foods like bloodworms to encourage your goldfish to breed. Once the chasing begins which usually happens early in the morning, you will find small tiny blobs of round-shaped eggs scattered everywhere in the tank. You should immediately remove the parents in order to ensure high survival of the eggs because goldfish are known to consume even their own eggs. A single female goldfish can produce close to several thousands of offspring at once and therefore do expect to obtain a large brood of fish fry in single spawn.
Goldfish eggs and fry development
After you have separated the parents, you should use an aquarium heater to bring up the water temperature once again back to normal (about 18 to 21 degree Celsius). Under this condition, the eggs will start to hatch in about 3 to 4 days and the tiny fry that emerge will cling on to the aquarium surface with its yolk sac still attached to its body. During this period, you should start preparing foods for them by getting ready your brine shrimp, which is considered, to best feed with high nutritional content. If properly cared for, the goldfish fry should achieve maturity in about 4 to 5 months period and start to develop colors going into the 6th month. During the fry development stage, make sure the water is periodically checked in order to obtain good water quality. (The above image showing fry at 1 day old moving to 2 weeks old and finally at 3 months old.)