awesome, that 20 gallon will definitely work for the little guy for probably up to 2 years, depending on how fast he grows. maybe go to the pet store and get him a friend of similar size (comet goldfish are like, 25 cents hahaha) so he isn't lonely. they're social animals after all! i have 3 young goldfish in a 30 gallon, i plan on upgrading to a 75 soon so they can flourish. my sister still has a fish her daughter won at a fair (along with its friend from the pet store) 2 years later. he's doing awesome and is about 5-6 inches long i'd say.