Unlike livebearers, such as goldfish, guppies can be difficult to breed. If you want to breed your Goldfish then you need to give them special care such as a big tank where they can find more spaces, good water, and proper nutrition.You can also need to notice water temperature changes. Before you stary goldfish breeding program you need to ensure that more spaces are available for them and their fries. If you don't have enough space for them then don't try to breed goldfish.
One of the most common cause of aggression and overcrowding. Ensure that you don't have too many goldfish in your tank. If you find that too many fish in your tank then you should find them for a new home. However, if your tank is large then you goldfish will fight each other if they are competing for food.