Pond fish generally start breeding in the spring or early summer. The rising water temperatures and lengthening days signal sexually mature fish to begin spawning.
Although common pond fish, such as koi and goldfish, spawn spontaneously in the spring, owners of ponds can promote early spawning by using a submersible heater to increase the temperature of the pond to about 68 to 74 degrees Fahrenheit.
After the eggs are laid, the adult fish may eat the eggs or fry. To prevent this, the adult fish or eggs should be moved to another pond. If this is not possible, a screen or mesh can be used to keep the adult fish and eggs separated.