Is that a fish bowl? A common goldfish is not recommend for tanks and generally need ponds. Fancy golfish need 20 gallons for one and 10 gallons for each additional fancy. All goldfish need high filtration as they produce a lot of waist
Commons are usually not recommended for tanks, but it's a ridiculous recommendation that makes no sense. It's possible to have a pond that is smaller than a tank. The recommendation should be a minimum size of container rather than its type.
The only reason for the recommendation is their size, but they're just as suited (or not) for tanks as any other fish they're size. Oscars and other large cichlids, piranhas and other large characins, catfish... there are many tropical fish that grow as big or bigger than goldfish, and these fish are never recommended as "only for a tropical pond, not a tank".