All of my fish seem to be dying at once. Any help?

Having six fish in that tank is crowded and due to the volume of water is likely to deteriorate quickly. Goldfish will produce a lot of waste and so even at that size still require a large volume of water. It is advised that you keep them in around 80 litres as jouveniles but will need to be converted to a large 120cm long tank or a pond preferable.