Best Time to Add Fish to a Pond

a group of fish swimming in a pond.

**Post contains affiliate links.* We are frequently asked this question, “when is the best time to add fish to a pond?” There are many factors that will determine if your pond and existing fish are ready for more pondmates.

Before Your Start – Do You Really Need More Fish?

In our 10 years in practice, we understand the burning desire to add fish to your pond or tank. Some owners will start out with one batch and never add more fish, while others just can’t seem to stop adding fish. Please keep in mind that adding more fish may not be the best choice for your pond. We recommend at least 250 gallons per koi and 50-100 gallons per goldfish. Other pond fish, such as catfish and high fin sharks, will also require considerable water volume, since they can grow and even exceed the size of most koi.

Even if you have the space, can your filtration keep up? If you are having difficulty with ammonia and nitrite levels that never seem to dissipate, these are clear signs that your filtration is struggling. You may just need to clean it out, especially if you have an old pressurized bead filter with no upstream mechanical filtration. Even after cleaning, if you cannot bring down your ammonia and/or nitrite levels, you will need to add more or rehome some of your fish. This is a clear sign to not add any more fish!

Before Your Add New Fish, You Better Quarantine!

The biggest issue we see with adding new fish to a pond is that they bring along a whole new host of bacteria, parasites and viruses. Again, in 10 years, we have yet to see a fish vendor actually take quarantine seriously, so beware of even the most “reputable” dealers. The best thing you can do to prevent a major outbreak in your main pond is to quarantine ALL new fish in a separate system for 4-6 weeks. Yes, we know it is a hassle. Yes, we understand you may not have room. But this is the most important thing you can do! This will save you time, money and lives.

It is recommended to salt your quarantine system to approximately 3 ppt to provide respiratory support and help treat minor parasite issues. There are no prophylactic medications that we recommend. Here are links to the salt we recommend and a salt meter to maintain your levels. Here is more information on how to properly set up a quarantine system.

When is the best time to add fish to my pond?

Okay, so you actually have room for new fish and your quarantine setup is good to go? Then let’s add some new fish! The best time of year to add fish will depend on where you live in the world, but for us here in California, late spring to early summer is the best time of year to add new fish to your pond.

Why is that? Well, in early spring, when temperatures fluctuate, outdoor fish are particularly vulnerable to pathogens floating around their environment. No fish is sterile, so your fish will absolutely have bacteria and parasites on them all the time. It is their immune system that will keep the pathogens at bay and your fish healthy. Koi immune function appears to improve around 60F or 15C. So, in the spring, your pond may bounce in and out of that range, causing the pathogens to multiply, but the koi immune response to lag. This is why we recommend spring health checks during this time of year to catch any issues early.

Therefore, the best time to add new fish to your pond is after this spring period, after temperatures have stabilized and your pond is consistently above 60F/15C. This will ensure your fishes’ immune function is operating at full function, so just in case anything slips past quarantine, your fish will not suffer.

Please also keep in mind that air temperature and water temperatures correlate, but are not exact matches, so we always recommend taking a water temperature daily to track changes and tell you more about your fishes’ appetite and behavior. Here is our recommended pond thermometer.

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