Whether you have an existing koi pond or an empty backyard water garden, you might wonder whether it would make a good home for the goldfish in your indoor aquarium. In most cases, ponds provide better habitats for goldfish than tanks.
Goldfish are natural friends of koi as they both are part of the carp family. Koi and goldfish share the exact water condition needs and adapt similarly to weather, making them perfect aquatic companions in an outdoor pond.
However, whether the fish in your aquarium can coexist with your koi depends on their size and other factors related to pond environment and maintenance.
Consider the Size of Your Goldfish
If adult koi already inhabit your pond, it’s essential to consider the size of your goldfish. Although goldfish can grow up to a foot, most kept in household aquariums only stretch a few inches. These fish need to grow to about 6 inches before it’s safe to place them in a pond with large koi. Although koi and goldfish can make great friends, both are cannibalistic species and will mistake their smaller brethren for a snack.
Goldfish that are too small will need time to grow in a larger tank or uninhabited pond with good water quality. Add protein to their diet to stimulate growth. Most fish foods formulated for growth include at least 50 percent protein. You can also buy gel foods that contain additional sources of protein, as well as growth-promoting vitamins and minerals.
Consider the Type of Goldfish
There are two classes of goldfish: pond goldfish and fancy goldfish. As the name implies, pond goldfish are well suited for pond life. While these fish are often packed into tanks at the local pet store or sold at fairs and carnivals, they require much more space than an aquarium can provide.
Fancy goldfish come in various shapes, colors, and fin types. They are smaller than pond goldfish but can grow to a suitable size for pond life if their environment is well-managed. They are less hardy than pond goldfish and don’t acclimate to the cold as well, so they are most appropriate for climates that don’t get freezing weather.
Consider the Pond Environment
First, you should ensure that your pond offers a suitable environment for goldfish. Like koi, goldfish produce a lot of waste. If you combine the two, you might need to increase the power of your filtration system.
A goldfish pond should have a large surface area, have plenty of aeration, and vary in depth (offering some shallow areas accompanied by areas at least 3 feet deep). It is crucial to ensure that the water does not freeze all the way through during the winter, so depending on where you live, your pond might need to be deeper.
Overcrowding is one of the main problems that arise within koi and goldfish ponds, so you should also consider the pond’s volume and the number of fish already in it. Keeping your population manageable avoids many potential problems like excessive waste build-up, depletion of dissolved oxygen, and stress from crowded conditions leading to disease.
In general, you should allow no more than two mature koi or three mature goldfish per 200 gallons of water. So, for example, if you have eight koi in a 1,000-gallon pond, you shouldn’t add more than three goldfish.
Consider the Water Parameters
Chances are that the water parameters in your pond differ from those in your aquarium. Many goldfish owners with small tanks or fish bowls rarely test the water’s parameters. Before transferring goldfish into your pond, you need to ensure that your pond has ideal water parameters and that the conditions in your tank match those for at least several days before introducing the goldfish to their new home.
Koi and goldfish are hardier than many other fish regarding their pH tolerance, but it is still important to keep levels between roughly 7 and 8.6. It is also essential to hold pH levels as constant as possible. Rapid changes in pH can stress the fish and, in some cases, be fatal.
Water hardness is the measurement of minerals dissolved in the water. Hard water has high levels of dissolved minerals and is usually higher in pH. Soft water has low levels of dissolved minerals and is generally lower in pH. In soft water, pH levels can change rapidly, but pH levels in hard water tend to be more stable.
Ammonia levels in a pond should be as close to zero as possible; the same is true for your tank. If your test kit detects ammonia levels higher than 0.1mg/l, change the water and add a beneficial bacteria treatment to the tank. The higher the level, the greater the percentage of the water you should change, while being careful not to shock the fish.
Nitrite levels also should remain close to zero, and the ideal range for nitrate is 20-60 ppm.
Consider the Water Temperature
Goldfish can survive in a wide range of temperatures but cannot withstand rapid changes. The fish in your tank have likely adjusted to a higher temperature than they will experience in the pond. The first step is to help them acclimate by gradually cooling their water to the pond’s temperature.
To do this, turn off the aquarium lights and move the tank away from any windows. Then remove the tank hood and replace it with fine netting so the fish can’t jump out. Aim a fan at the tank to gently blow across the water’s surface and float ice packs in the water. But remember to go slowly–the water temperature should not drop more than 2 degrees every eight to 10 hours.
The best time to transfer goldfish from a tank to a pond is typically in the summer when the aquarium temperature will be closest to that of the pond. After matching the temperatures, float the aquarium fish in a plastic bag with plenty of aquarium water for a few hours when you first place them into the pond. This extra step will help the fish adapt to any remaining temperature discrepancies.