It's often worthwhile to place netting over your fish pond to keep it from collecting leaves and detritus in winter. It can also protect your fish from predators, especially when there are no plants to hide behind and their metabolisms have slowed so they can't swim fast enough to escape. However, covering a fish pond with a solid cover could prevent natural air exchange, preventing toxic gases from escaping and oxygen from entering. This will quickly kill your fish.
Some owners may want to protect their ponds from rain and snowfall. It's possible to leave a gap around the bottom to allow air to flow, but snow is heavy and even small accumulations can cause the structure to collapse and inflict even more damage to your fish. In a well-designed, properly maintained pond, mesh type covers can be a excellent protection from predators, but solid winter covers shouldn't be used. Cold water fish and plants will do just fine when they're well prepared for the season.
