the general rule is, 20 gallons for one goldfish, 10 extra gallons per extra goldfish. so, for two goldfish, you'd need about a 30 gallon tank. for five, you'd need a 60 gallon. common goldfish, need bigger spaces. the need, if i'm not mistaking, 10 times the filtration, as they have tank size(i could be VERY mistaken on that, though). i won a common goldfish at a fair, and he was just a few inches long. 11 or so years later, he done ate all his tankmates except a pleco, and they lived happily in our 29 gallon tank. he died, though, and was almost 2 feet long. if my dad hadn't dumped random pond and lake fish into the tank randomly, he may still be with me today. they live over 20 years. :d and, i'm turning 24 this year.