The clearest sign that goldfish are mating is when a so-called spawning chase occurs. This involves the male goldfish swimming alongside the abdomen of the female goldfish for a few hours before the female commences laying eggs.
Once the eggs are laid, the male goldfish will immediately spray milt on them to fertilize them. Most females lay from 500 to 1,000 eggs, in batches. Breeders usually remove the fertilized eggs from the tank and place them in another tank for hatching. Keep in mind that to facilitate spawning, keeping the tank at the optimal temperature (50 to 78.8 degrees Fahrenheit) can expedite the process and encourage spawning. Notably, goldfish tend to spawn more often in the light of early morning than at any other time.