Fancy goldfish need 10 gallons per fish while common and single tail goldfish need 20 gallons per fish. The fish need more space, and they need to be around 3 years old before they are old enough to breed. The fish will be around 4 to 5 inches long w/o tail when they are the right size to breed. A bowl is not the proper home for a goldfish they are too large and messy of fish to live in a bowl. Once they breed you will have around 100+ fry (baby goldfish) that will be 1,000+ gallons of water that will be needed to raise them.