Def agree with Colin. Goldfish get way too big. You can keep them in that size of a pong for a while but eventually they will have to go to a bigger sized tank or pond.
I would say you could do guppies or a smaller fish. Depends on temps imo. I have an outdoor 40 GAL pond and can't really keep anything during the summer due to the heat in Houston. I'm thinking one day of getting a chiller for it but not sure if feasable (if it can keep up with the heat from the sun).