The usual formula for calculating whether goldfish have enough space is one inch of fish per 24 square inches of surface area (so 48 square inches for a two inch fish) or one inch of fish per gallon of water (2 inches per 2 gallons). Another simpler calculation is roughly four gallons of water per fish. These calculations are for tanks WITH A FILTER. Never keep a fish in a tank without a filter. It is unnecessarily cruel. The fish are far more likely to die. It’s best to buy a tank with space for future goldfish growth.