How to Keep Aquarium Fish in Outdoor Fountains and Ponds

Ideal Conditions and Fish Species for Your Outdoor Pond

Garden pond with koi fish

ViewStock / Getty Images

Many common aquarium fish don't have to be kept in the controlled environment of a heated tank. Instead, some aquarium fish can live outside in certain environments to benefit backyard ponds and fountains.

Having active, colorful fish in fountains and ponds isn't just a fun spectacle. It can solve several problems with outdoor bodies of water. The fish often will feed on undesirable insect larvae in the water. You'll also be less likely to get cloudy water from algae and other materials the fish will eat. But you have to know how to keep your fish happy and healthy outdoors.

Ideal Conditions for an Outdoor Aquarium

Many aquarium fish that come from temperate and subtropical climates will thrive and breed in outdoor fountains and ponds if certain conditions are met.

  • The average water temperature must be consistently over 65 degrees Fahrenheit at night and over 72 degrees Fahrenheit in the daytime. When the weather starts getting cooler, the fish need to be brought indoors.
  • A pond should be at least partially submerged in the earth, which helps to normalize water temperature. A fountain with a substantial wall thickness in the pool portion also could work.
  • The pond needs sufficient plants, so the fish can seek shade from intense sun.
  • The water level has to be kept up with either rainwater or aged water, not water directly from a garden hose unless a de-chlorinator is added. However, overspray from a sprinkler system should not hurt the fish in small amounts.
  • The pond should have a filter to remove waste, and you'll need to make periodic water changes to keep the nitrate level down. If algae becomes a problem, try an ultraviolet filter.
  • Grass clippings should not be allowed to fall into the pond, as they often contain lawn chemicals that could poison the fish and pond plants. Cut grass, leaves, and other debris will also decompose quickly and contaminate the water, making it cloudy and increasing the ammonia level.
  • Feed the fish once a day with commercial fish food, similar to what you would feed them in an aquarium.

Best Fish for Outdoor Aquariums

Some of the oldest varieties of aquarium fish are called tropical fish, but they come from temperate climates and survive very nicely in unheated aquariums. The paradise fish and the White Cloud Mountain minnow are good examples of hardy species that do not need much warmth.

Goldfish and koi long have been good pond fish, as they can even handle water icing over the top of the pond in the winter. But they must be protected from predators and should be kept in deeper water than smaller fish. Moreover, goldfish and koi are not effective at controlling mosquitoes, and they produce a substantial amount of waste, requiring more maintenance and filtration than smaller aquarium fish.

Other species that could work for your outside pond project include the guppy, molly, swordtail, blue gourami, cory catfish, and plecostomus.

White cloud mountain minnow (Tanichthys albonubes)
Bruno Cavignaux / Getty Images

Acclimating the Fish

All fish must be properly acclimated to the outside pond water. If you already have an established pond, it's ideal to make a bucket of half pond water and half aquarium water. Let the fish get used to that water for about an hour, using an air stone and air pump to aerate it. 

Regardless of whether you do the water acclimation step, you must put any fish in a plastic bag of the water they are coming from and float it in the pond or fountain for at least 15 minutes before releasing the fish. This gradually equalizes the water temperature between the bag and the pond, so the fish isn't shocked by a temperature change in its new environment.

Guarding Against Outdoor Predators

Although the fish suitable for outdoor ponds are hardy, you still must lookout for predators. Some species, such as guppies, platys, swordtails, and mollies, are such prolific breeders that the constant addition of new fry will outpace just about any predator. But some of the larger fish, including goldfish and koi, might see their numbers dwindle due to attacks from cats, raccoons, birds of prey, and more. 

Provide your fish with plenty of places to hide, such as caves at the bottom of the pond and plant cover. You also can use netting over the pond as well as decoy statues that scare away the predators.

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  1. Lawns and Landscapes in Your Watershed. Massachusetts Department of Environmental Protection.