keeping a goldfish in anything smaller than what's recommended is a death sentence for them. :I i had my fancy goldfish in a 10 gallon tank with a friend, because i was told that was okay. kept up with water changes. they still died. my common goldfish would have lived longer than 11 years if i'd had him in a pond, rather than a 29 gallon tank. the recommended size is just that, because it gives the fish room to grow and dilutes the ammonia AND gives them room to grow. a foot-long goldfish isn't going to be happy in a 10 gallon tank. there's no room to swim for them.