Koi Pond
There are lots of decisions to make when planning a backyard pond: the size, the location, the landscaping. But before you go too far, don’t forget the most important question: Who’s going to live there?

Whether you’re interested in keeping ornamental fish or attracting wildlife — or are hoping to do a little of both — you need to think ahead about the needs of these creatures.

Pond Basics

Any backyard pond will need access to electricity. You may be able to do without filtration if you’re not going to have fish, but you’ll still need a pump for a water feature like a waterfall or fountain. Even for native wildlife, you can’t just dig a hole and fill it — the water needs to be in motion. "Natural bodies of water do have a flow," says David Mizejewski of the National Wildlife Federation. "Even something that doesn’t look like it’s moving, like a marsh, has a flow of water."

A moving water feature helps with aeration, and if you live in a cold climate it will also keep the surface of the pond from freezing over completely in the winter. Both are important for maintaining water quality.

Any pond should also be big enough not to freeze solid. "Even for amphibians you want to make sure it’s deep enough not to freeze all the way to the bottom," says Mizejewski. Frogs will hibernate in the litter at the bottom of a pond, and with the exception of the wood frog, they can’t tolerate freezing.

Which Fish?

Goldfish and koi — those large ornamental carp beloved in Asia — are related, but have some differences that may surprise you.

It’s more difficult to keep aquatic plants with koi, which tend to dig around and disturb roots. Goldfish are much more plant-friendly, and plants are important in a pond for more than aesthetics. "Plants help filter and clean the water and shade it so you don’t get algae blooms," says Mizejewski. The result is that with koi, more sophisticated water-filtration systems are usually necessary.

Another difference is that goldfish will stop growing before they get too big. "They’ll grow to fit their environment," says Jon Sander of Lilypons Water Gardens in Maryland. Koi, in contrast, keep growing all their lives, so unless you have a large pond, you’ll need to have a plan for when they get too big. (For this reason, Lilypons has a "retirement pond" where people can return their fish if necessary.)

Fancy varieties of goldfish will generally do fine in a well-kept pond, says Sander, with one warning: They’re more vulnerable to predators. Even in a city, your pond may attract raccoons, fish-eating birds like herons, and even snakes. "The more bulbous, less-streamlined fish are going to be easier to catch because they can’t swim as fast." So if you go beyond the basic model of goldfish, make sure to provide plenty of hiding places.

Going Wild

If you’re hoping to attract native wildlife like frogs and turtles to your pond, they’ll need a way to get in and out of the water. When building a new pond, you can incorporate that into the basic design. "Create a shallow graded shoreline, so animals can get in and out," says Sander.

An existing pond may not have this feature, but you can place plants around the edges to provide access routes. "Even better, have some kind of branch or log that sits on the shore and extends down into the water so animals can climb in and out that way," says Mizejewski.

If your main priority is to help out endangered amphibians, you may want to leave out the fish. While it’s certainly true that fish and frogs coexist in the wild, not all frogs actually reproduce in the presence of fish. Some species breed in what are called vernal ponds, which are spots that fill with water only in spring, where fish can’t survive. "Many different species have evolved to breed in these tiny, ephemeral, fishless vernal ponds specifically to avoid that level of predation," says Mizejewski.

If you do have fish, provide plenty of hiding places, including plants. While the fish will probably end up eating some of the tadpoles, if the pond is large enough, some will survive.

Keeping Healthy

There’s not much risk of fish making wild amphibians sick, says Dr. Greg Lewbart of North Carolina State University College of Veterinary Medicine, because they don’t share a lot of diseases. "The good thing about fish is they tend to keep their problems to themselves," he says.

However, new fish can bring diseases to your existing fish, so Lewbart recommends you quarantine them in a separate location for a month before adding them to your pond. "It doesn’t have to be pretty or fancy," he says; goldfish will be fine in a five-gallon bucket with an airstone and frequent water changes.

Your pond is also not likely to breed certain annoying carriers of human disease: "The research shows that bird baths and water gardens are not significant contributors to local mosquito populations," says Mizejewski.

In a pond with moving water and plants covering part of the surface, there’s not much opportunity for mosquitoes to lay eggs. And you’re actually better off not using chemicals to kill them, which are also bad for the wildlife that eats mosquito larvae. "If you’re putting in chemicals, you might be eliminating the mosquitoes, but you’re also keeping the natural checks and balances out as well," says Mizejewski. "And the mosquitoes are the ones that come back first."

Finally, protect the health of your pond in the same way you would any other pet: Before you purchase your fish, check to see if there are any local veterinarians who can treat them should they become ill.

Put Out the Welcome Mat

Being wildlife-friendly means letting the wildlife come to you. Don’t buy frogs and turtles for your pond, and never let your fish get into natural bodies of water. "This is how eastern bullfrogs got introduced to the west, where they are an invasive species," says Mizejewski. Released in people’s water gardens, they’re now eating some local amphibian species into extinction.

And don’t go out catching creatures from the wild and bringing them home either. You can’t be sure you have the habitat a particular species needs to survive, so let the animals decide. Mizejewski says, "The real message here is, if you build it they will come — if you do it right."

A note of precaution: Some wildlife that your pond may attract can carry parasites that can be passed to pets and people. Talk to your doctor and veterinarian for advice on how to help protect your family.

Read more Vetstreet articles featuring wildlife:

Help Endangered and Threatened Wildlife by Taking Action, Going Green

Wild and Exciting: Inside the Unconventional Lives of Wildlife Photographers

Two New Apps Provide Real-Time Wildlife Sightings at Yellowstone National Park

Why You Should Think Twice About Trying to Rescue Baby Wildlife