Garden ponds can be kept in most parts of the world, and if you select cold tolerant species you can have a pond filled with thriving fish even if you live in one of the colder climate zones. Some fish species will even tolerate freezing temperatures, but if the pond freezes over completely they can suffocate since no gas exchange can take place between the water and the surrounding atmosphere. In some regions, it is therefore better to house the fish indoor during the coldest winter months or use pond equipment to make sure that the pond is kept open and oxygenated even during the winter. Some fish species have developed a method of hibernation where they stay dormant during the winter and consume very little energy and oxygen.
A common mistake among beginner pond keepers it to create a small pond thinking that a larger pond will be too hard to maintain. In reality, a large pond is however much easier to maintain than a small pond. Experienced aquarists recognize this phenomenon from the aquarium world. In a small aquarium, a small amount of left over food or a single dead fish can be enough to pollute the entire aquarium. In a large aquarium, the large water mass will dilute any pollutants and make the concentration of harmful compounds low enough for the fish to tolerate. In a pond, a large water mass is even more important, since pollution can enter from the surrounding world. When you create an outdoor pond, you should also keep in mind that a big pond will create a more stable environment for your fish temperature wise. A large and planted pond will also make it easier for the fish to hide from predatory birds, cats and other animals. Another important factor to take into consideration if you plan to create your own pond is location. A shading tree will be nice for the fish, but will on the other hand force you to regularly remove fallen leaves from the water.
The two most frequently kept pond fishes are Goldfish and Koi. Koi feed on plants, and can therefore only be kept with a smaller selection of plant species. Goldfish will typically leave the plants alone, and can therefore be kept with virtually any aquatic plant species. Another difference between Goldfish and Koi is that Koi grow larger and need a bigger pond. If you wish to keep Koi, you will need a pond of at least 1000 gallons (3785 liters). A Koi pond should also be 3 feet (90 centimeters) deep or more. A Goldfish will be fine in a pond that is no deeper than 2 feet (60 centimeters), unless you live in a cold climate. If you want your Goldfish to survive in the outdoor pond during a cold winter, you must provide them with at least 12-16 inches (30-40 centimeters) of water below the freeze zone. The deeper the pond, the safer your fish will be. If you have a shallow pond, you must house your fish indoors during the winter.