Instructions
Remove and discard any eggs you find in the tank. You can find them on plants in your tank. Or, just leave them in there and let them hatch, as the parents will probably just eat them.
Separate males and females and place them in different tanks. The genders are a bit difficult to tell apart, but female goldfish are usually more plump than male. Also, males have small bumps or "tubercles" on their heads that will turn white when he is ready to breed.
Lower the temperature in the tank below 20 degrees Celsius -- 68 degrees Fahrenheit -- which is the optimum temperature for breeding. Also, the eggs need the water to be 21 degrees Celsius to hatch.
Put the fish in a tank that is smaller than 15 gallons per fish. Goldfish are most likely to spawn when they have adequate space.
How to Keep Goldfish From Breeding
If you've got a few goldfish of different genders in your fish tank, you might be worried that they'll suddenly start spawning and creating a whole bunch of problems with a crowded and dirty tank. However, it's actually difficult to breed goldfish, and even if they do spawn successfully, the males will usually gobble up the offspring. If you still want to take some extra steps to ensure that you don't end up with goldfish fry, you can implement a few methods to discourage the fish further.