It's very rarely a good idea to completely drain your pond for the winter. In fact, there is no compelling reason to drain the water in either a fish or planted pond, as long as you winterize the pond correctly. On the bright side, your pond may end up healthier and happier after a good winter's nap. Of course, the first place to start is to check the type of fish and plants you have and consider whether they're hardy for the winters in your area. If ponds typically freeze over in your region, tropical fish and many other types simply won't make it. Move your tropical fish indoors to an aquarium for the season. You can store your tender aquatic plants (which are typically annuals) the same way, but many pond owners choose to compost them and start fresh in the spring. For your cold-hardy friends like koi and standard (not fancy) goldfish, you'll need to make sure the pond is deep enough to keep them healthy during the coldest stretches. A minimum of 3 to 4 feet should be fine.
To provide the best possible winter environment for your fish and plants, plan to do up to a 50% water change. This will start your pond off on a good foot (or fin) before the winter begins. Before the water change, remove any sludge or debris that has collected at the bottom of your pond to reduce the amount of decay and its byproducts that will be trapped in the pond under ice. Remember to stop feeding your cold water fish as soon as the water temperature drops to 50 degrees and below. Your fish will go into torpor and cannot digest food when their metabolism slows drastically, so continuing to feed them past this threshold could actually kill them. Your pond plants should be cut back once the foliage is damaged by frost, and the pots submerged in the deepest part of the pond.
