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What to Do When Your Fish Grow Too Large

What to Do When Your Fish Grow Too Large

Have you heard the saying a fish will only grow to the size of their tank? This myth usually applies to goldfish, who often end up in tanks and bowls that are too small. If there is not enough space for your fish to grow to its full size, it will likely develop health problems and live a shorter life. Koi, goldfish, and other pond fish may be small when you buy them from the store, but they can actually grow quite large. You have options if your fish outgrow your pond and it becomes overcrowded. 

Goldfish and koi are popular pond fish because they are colorful, friendly, and hardy. Small goldfish are often given away at carnivals and kept in indoor tanks (without a pump), and only live for a few months and stay small. Goldfish can grow large and live for 5 to 10 years in proper conditions. The Guinness Book of World Records winning goldfish was almost 19 inches long. 

Koi can easily grow 3 feet long and live 25 to 30 years. When you purchase koi or goldfish from the store, you need to keep in mind how large they will grow as adults. 


Ideally, your pond should have 10 gallons of water for every inch of fish. Fish need plenty of clean water to swim in and be healthy and happy. When a pond becomes overcrowded, there are too many fish. Fish produce waste containing ammonia, which is toxic to pond life in large amounts. 

In an overcrowded pond, fish waste can become more than the pump and filtration system can handle. The waste sinks to the bottom of the water to be broken down by the bacteria. If there are not enough beneficial bacteria, then the waste is turned into harmful gases. 

During the summer, the oxygen levels tend to fall as the water warms up and the algae battle begins. Your pond fish become more active, needing more oxygen. This can cause the oxygen levels to fall and give the algae a free pass to take over. When algae take, oxygen levels fall further and can even cause beneficial bacteria to die off. 

An overcrowded pond is not the ideal environment for healthy and happy fish. The stress makes them more vulnerable to illnesses and parasites and can even cause fighting, as they have to compete for space. 

How to Tell if Your Pond Is Overcrowded

The rule of thumb is 10 gallons of water for every one inch of fish. It may be a little difficult to estimate how many inches of fish you have in your pond unless your pond is small and not too deep. There are a few signs of overcrowding that can be a red flag. 

  • Poor water quality
  • Fish stress & sickness
  • Excess waste (smelly water)
  • Changes in behavior
  • Fish fighting
  • Algae

Overcrowding is typically accidental. You may underestimate how big your fish will grow when you buy them as juveniles. Your fish may even reproduce, and you could find yourself with new fish that you didn't even buy. 

What Can You Do?

If your pond is overcrowded, you need to either make your pond bigger or find some of your fish a new home. Pond fish, especially koi and goldfish, tend to be a little more friendly and enjoy greeting their owners like other pets. They're intelligent and become part of your daily routine. 

A common misconception is that you can set your fish "free" in a local freshwater pond or stream. Common pond fish are not native to your area. You do not see "wild" goldfish or koi in North America. They are descendants of carp, which are less colorful and native to China. Introducing non-native fish and plants into the wild disrupts the ecosystem. The goldfish and koi can overpopulate, taking the resources away from the native fish. The natural predators may be unable to eat larger koi and goldfish, so they continue to populate. A handful of goldfish can take over a lake in a few years, and then a local Parks and Wildlife center will have to intervene. 

Make Your Pond Bigger

Once you add the liner, plants, and other decorations, your pond ends up smaller than the original hole you dug. If your fish are overcrowded, this could be an opportunity to expand your pond. It will take a little work, and you may need to locate your fish in a temporary tank during the process. You also may need to upgrade your pump

Add a Second Pond

You can add a second pond if you have extra space. Above ground ponds can be easier to set up than in-ground ponds and leave a lot of room for creativity. You can use an antique bathtub or trough and turn it into a rustic pond to house your growing fish; just make sure you have a pump to keep the water oxygenating. Learn how to make an easy DIY above ground pond


You can re-home some of your fish if you don't want to expand your current pond or add a second one. Adult koi and goldfish, especially less common color varieties, can be quite expensive. You can sell or give away your koi or other fish. Find a local koi, pond society, or club to find a new and loving home for your fish. Tips for sharing information about your fish:

  • Clear photos that show the fish
  • Photograph any koi with special markings
  • Know as much detail as possible on health and where you got them
  • Accuracy, make sure you know the difference between your koi and goldfish

You may be able to find a local fish rescue and shelter to take your unwanted fish. You can also check local aquariums, gardens, and pet stores to see if they accept fish. Local goldfish and koi breeders may be interested in purchasing your unwanted fish if they are healthy and have unique color patterns. 

Overcrowding is a difficult issue because your fish become pets, and you want to give them a good home. Koi and goldfish start small, and it's exciting to add new additions to your fish family, but they grow and sometimes reproduce over the years. When your pond is overcrowded, the water quality drops, and fish become stressed and sick. Releasing your extra fish into the wild harms the ecosystem and creates a problem for the local wildlife. You can create new space for your fish or find them a new place to live. You can find the perfect new home for your fish if you're patient.

Visit for information on best practices when rehoming fish or other animals. 



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