Goldfish require plenty of space. The bigger the glass aquarium tank, the better. You’ll need at least a 20-gallon long tank for two Goldfish.
However, since Goldfish can grow anywhere between 6-inches and 2-feet long, your pair may outgrow that size. It may be better to choose a larger tank size to begin with, such as a 40-gallon or 75-gallon aquarium tank. If you are trying to breed your Goldfish, a 40-gallon breeder tank may be appropriate to use at that period to help keep them in close proximity.