The general rule of thumb is one Goldfish for each 10 gals of water at a minimum so you really can't add another fish. Goldfish create a lot of waste so they need quite a bit of water per fish. Besides, Goldfish are not tropical so they like water cooler then what tropicals require. A Goldfish tank is just that, one species of Goldfish in the tank. For example, if you had a larger tank and you had a Fantail Goldfish you would only add other Fantails. Different types of Goldfish grow to different sizes and swim at different speeds. As they get larger the fastest Goldfish or the more aggressive species will get most of the food.
"We're sorry Moe, we thought you was a fish!"
Amalgamated Association of Morons, Local 6 and 7/8's