When is the Best Time to Buy Pond Fish? (When to Stock Fish)

When you want to incorporate new fish into your outdoor pond, you have to be mindful of the season and your location. Most common pond fish, particularly juveniles, are exceptionally sensitive to changes – such as the water temperature, pH, and just a new environment in general – and should only be introduced during the proper season and under suitable conditions to ensure that they are able to safely acclimate and survive.

By definition, fish are poikilotherms, meaning that they can’t really regulate their own body temperature but rather are reliant on ambient water temperatures to determine their body temperature. Typically, their body temperatures are .5 to 1 degree Celsius within the range of the surrounding water temperature, and this is a very important point to keep in mind when adding new fish to ponds.

In this article, we will go into more detail about the best times  to buy and stock fish in your pond to reduce the chances of causing them thermal shock or any illnesses that become more likely when fish are stressed.

When To Stock Ponds With Koi, Goldfish & Others Species

Typically, you can begin stocking ponds in the spring once water temperatures begin to increase over 50° F (10° C) and can continue throughout the summer but must stop again in the fall when temperatures drop. If your pond is new, you should stock only a few fish at a time to allow your filters to build up some beneficial bacteria, waiting a week or two before adding any more fish.

If your water temperature is more than a couple of degrees warmer or cooler than the water that the fish were previously kept in, try placing them in a bag or container with the original water they were previously kept in. Place that baggy or container in your pond for about an hour or two to allow the water inside to gradually adjust to the temperature of your pond so as to not shock your fish when you place it directly in the pond.

When you stock your pond will also depend on the species that you are looking to buy. For example, koi shouldn’t be stocked if water temperatures are below about 60° F (15.5° C), sturgeon are cold-water fish and can be stocked as soon as water is 50° F (10° C), and fancy goldfish (depending on the exact variety) really shouldn’t be stocked until temperatures are in the mid 60’s or higher (18 or more in Celsius).

Be sure to research the exact fish species as well as variety that you wish to purchase so that you can know its exact water requirements and thus give it the best chance at long term survival in your pond.

Can You Add Fish to Ponds in Winter? (When NOT to Stock Ponds)

Unless you live in a temperate area in which your water doesn’t drop below 50° F even in the winter, you should never stock ponds in the winter as your fish will most certainly die – this is also why many less hardy fish should be brought indoors for the winter. Similarly, you shouldn’t stock fish during periods of extreme heat or when water temperatures exceed 75° F (about 23° C). Not only will the water temperature directly shock them, but temperature influences every other aspect of water quality as well.

For example, colder water (until it gets below freezing) holds more dissolved oxygen – fish that have already been in your pond for a while and are cold-tolerant will simply congregate to the water at the bottom of the pond that has more oxygen. Due to winter turnover, water with little oxygen will be located toward the top of the pond (particularly in larger ponds or lakes), meaning that new fish will be exposed to this region and could suffer before they get to the more oxygenated region. Water that is too warm similarly holds much less dissolved oxygen, and most fish need at least 6 parts per million of DO in order to survive, though 8 or more is best.

Cold winter temperatures mean that most, if not all, of your aquatic plants will die, which also leads to reduced dissolved oxygen, less filtration, and fewer beneficial bacteria. Furthermore, as water temperature decreases, pH increases. More alkaline (high pH) water kills off the bacteria that break down ammonia; without these nitrifying bacteria, water can become toxic to fish. Warm water, by contrast, results in acidic pH and can lead to algal blooms if you’re not careful (another reason to not stock if water temperatures exceed 75 or 80° F for an extended period of time).

Preparing Ponds Before Buying Fish (Very Important)

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Regardless of when you add fish to your pond, the water should be cycled first to build and maintain a proper ecosystem for the fish. The fastest way to do this is to purchase beneficial bacteria supplements that contain beneficial bacterial strains from most pet or water supply stores.

You can also add small amounts of waste from fish and plant matter, or some fish food, to really launch the nitrogen cycle, which is essential for properly breaking down waste and other potentially harmful compounds into forms that are usable by plants, beneficial algae, and even fish. You’ll then need to circulate the water through your filter(s) and pump(s) to make sure that everything is well mixed while also removing any potentially harmful chemicals like chlorine or lawn fertilizers.

You should allow this process to occur for a couple of weeks before adding fish, testing the water quality (such as temperature, pH, hardness, oxygen levels, ammonia levels, and so on) daily to see if any changes need to be made and make sure that the conditions are healthy enough for fish. Adding plants will also add to the pond’s ecosystem and directly influence water quality while also providing potential food and shelter for fish, along with a bit of temperature control during the warmer months as they supply shade.