It should be water, but it won't lack oxygen unless the fish tank is small. Before buying goldfish, put the fish tank in water for 2 days before buying fish. Don't rush to feed the goldfish you just bought. It's okay to be hungry for a few days, let it adapt first, but it's easy to die if you feed it in a hurry. Don't change water too often in winter. It doesn't matter if you change water from one week to ten days. The replaced water should also be placed in the same room with the fish tank a few days in advance to make the water temperature similar to that of the fish tank. Change only about one third of the water, not all. Just change the fish excrement at the bottom, then suck up the dirty things at the bottom with a straw, and then slowly pour clean water down the wall of the fish tank, not at the fish, let alone directly on the fish.