You definitely need to move them to the 50. First, common goldfish grow very large and are very messy fish. They need all the space they can get and excellent filtration. A pond is the only real place for them so I would start on one in the spring if you can. Second, the temp of the water will swing more drastically in the 10 than the 50. Although they can handle cold temps easily, the swings you'd see from day to night in a 10 will stress them.