Raising goldfish can be a rewarding experience, but an aquarium can easily become overcrowded when the fish are allowed to spawn and procreate at will. This can leave you with a difficult decision: Add a second tank that you will have to take care of, or somehow get rid of the baby goldfish. The easiest way to avoid this dilemma is to take steps to prevent the goldfish from reproducing.
Check the tank's thermometer. The normal temperature for goldfish is 18.3 to 20.0 degrees C. Water temperature of 21.1 degrees C or warmer induces spawning. Lower the tank temperature immediately.
Change the water less frequently. Goldfish spawn more readily when the water is fresh, so if you normally change 20 per cent of the tank's water every few days, change it less often.
Use the magnifying glass to check the sex of your goldfish to avoid male and female pairs in the same tank. Determining the sex of goldfish is more difficult when the fish are small, but spawning usually starts when they are ages 2 to 3. Mature males will develop tubercles or whitish bumps on their head or gill covers.
Watch your goldfish. When spawning starts, the male will chase the female around the tank and ram into her many times. He does this to expel her eggs so that he can fertilise them. If you observe this aggressive behaviour, use a net to transfer the female to a fish bowl by herself for about a week. The separation will break the spawning cycle and help keep her safe.
Look for eggs in your tank. Remove any eggs immediately before they hatch into fry. If there might already be eggs in the tank, add a few bottom-feeder fish to eat them.
During a spawn, fish can become protective and aggressive toward other fish in the tank.