I would get at least a 55 gallon or bigger tank for goldfish that size (they will probably be bigger by the time you bring them in, too). Or, if like me you can't afford a 55 gallon or bigger tank, you can get a really big plastic tub or tote or even kiddie pool for them. They're cheaper, don't need a stand, and are designed to be able to hold that much water. We've got a big, long plastic tote that we're bringing my grandmother for her pond fish during the winter, should be able to set it on the floor whereever she wants it and fill it with water from the pond as we drain it.