How Many Fish Will Fit In A Backyard Pond Per Gallon?

a large group of fish swimming in a pond.

Do you want to add fish to your backyard pond but aren’t sure how many to add? Do you want to make sure your fish are safe and happy in their new home?

In this post, you can find the answer to “how many fish can I have in my pond” and more! By the end of this article, you should know what to consider when stocking your pond and how many koi to add to your pond.

How Many Fish Can I Have In My Pond Per Gallon?

Here are two basic rules for determining how many fish fit in a pond per gallon:

  • One 6-inch fish for every hundred gallons of water
  • 1 inch of fish per 10 gallons of water

Testing The Fish Per Gallon Rules

Let’s test these rules using the average size backyard pond that’s built as a self-supporting ecosystem.

Calculating the water volume: 10 feet (length) x 15 feet (width) x 1-and-a-half feet (avg. depth) x 7.5(gal/cu ft) = 1,687.5 gallons. Multiply that by .75 (since the pond isn’t exactly rectangular and has rock and gravel, which accounts for some of the total volume).

This means that the true volume of our 10-foot by 15-foot pond is about 1,266 gallons. Using the two rules of fish per gallon, the results would be:

  • 12.7 6-inch fish or 76.2 inches of fish
  • 126.6 inches of fish or 21 6-inch fish

How Many Fish Can I Have In My Pond Per Inch

1 inch of fish for every square foot of surface area in a 2-foot deep pond is the standard rule. Using the above numbers, the number of fish per pond would be 112.5 inches of fish (using the length x width x .75).

These rules are standard for ponds, but they can be broken. Ponds with extra filtration, more aeration, and bogs can support more fish.

The 1-inch of fish per square foot rule applies to new ponds, but after the pond establishes itself (biological activity occurring), it might support 2-3 inches of fish per square foot. Add them gradually to make sure the pond ecosystem can handle them.

What To Consider When Adding Fish To A Pond

There are several factors to think about before adding fish or more fish to your pond.

The Ponds Total Surface Area

Surface volume is the amount of water contained in one cubic foot or meter of water from the surface to the bottom of the pond. This measurement is important when calculating how many fish can be safely added because it takes into account both depth and size.

When it comes to creating a habitat for your fish, the surface area of the water is just as important as its volume. This is because, unlike the deep waters of large lakes or oceans, most backyard koi ponds don’t have enough surface area for enough oxygen and other essential nutrients to be exchanged between air and water. As a result, even if your pond has plenty of water volume (in other words, it holds plenty of gallons), it may not have enough surface area for your fish to thrive. 

Surface volume affects more than just oxygen levels; it also impacts how quickly nitrates accumulate in the water. Nitrates are compounds that form when organic matter breaks down in an aquatic environment. Too many nitrates can lead to algal blooms, choking out beneficial plants and animals in your pond.

Pond Volume

The volume of a pond is determined mainly by its shape and size, as well as its depth. When looking at an oddly shaped pond, it can be helpful to break it up into sections and calculate each one separately. This will help ensure that you get an accurate reading when calculating the total pond volume. If your calculations are off, then you may end up overcrowding the pond or not stocking enough fish. 

Pond Size

When it comes to size, it’s important that you don’t buy enough fish to fill your entire pond with one species. Different types of fish require different sizes of ponds, so it’s essential that you research each type before adding them to your pond. 

A bigger pond means more space for aquatic life. However, more fish also means more maintenance, more water changes, and more filtration systems must be installed and maintained regularly. Smaller ponds require less upkeep and fewer changes over time but will limit how many fish you can safely add due to the smaller available space. 

The Number And Type Of Filtration System

Filtration systems are essential for maintaining a healthy koi pond environment. A filtration system helps keep the water in your pond clean by removing waste particles (like fish droppings or excess food) as well as ammonia, nitrates, and other potentially harmful chemicals that can build up over time. This means that when it comes to adding fish to your koi pond, it’s important to consider not only how many fish but also what kind of filtration system is in place. 

Flow Rate

Knowing your flow rate is important when adding fish to your koi pond because it helps determine how many fish can safely inhabit your pond without overburdening its filtration system. For instance, pond owners with a medium-sized koi pond with an undersized filter system may need to limit their fish population so that fish waste doesn’t overtax the filter’s capacity. On the other hand, if you have a large pond with a powerful filter system, you may be able to introduce more fish since they won’t produce enough fish waste to overwhelm your filtration capabilities. 


Water temperature is an important factor for all kinds of aquatic life. Fish, in particular, need temperatures that are within a certain range to survive and thrive. If the water gets too hot or too cold, it can cause stress on the fish, which can lead to disease or even death. For this reason, it’s important to pay attention to the water temperature when setting up a koi pond. 

pH Level

The pH level in water affects the amount of oxygen available for aquatic life forms like fish. When water has too much acidity or alkalinity, it robs the water of oxygen, leaving your fish struggling to survive. Generally speaking, most species prefer slightly alkaline water with a pH between 7 and 10; however, some species may prefer slightly different conditions depending on their native habitat.


Koi, like all other pond fish, have optimal temperature ranges that they can tolerate. In general, most koi species do best when water temperatures are between 65-75 degrees Fahrenheit. Therefore, if you live in an area with cooler climates, such as the Midwest or Pacific Northwest, it is important to consider how cold the pond water can get for a prolonged period of time and whether such temperatures are suitable for the koi species that you want to keep. You may need to install a heating element or use a deicer during particularly cold winters in order for your fish population to thrive.  


When it comes to growth, leave some breathing room for each type of fish so they can grow to their full size without crowding out other species or overcrowding the entire ecosystem. Make sure not to count their tails when calculating size; this is especially important when considering koi since their long tails can take up more space than anticipated! Be sure to leave room for 1-2 inches between each individual fish as well. This will give them enough space for proper growth over time. 


Fish are like underwater rabbits- they have a lot of babies. They normally spawn during late spring or early summer when the water is warmer. It’s possible you may end up with baby koi swimming around in your pond unless you get same-sex koi.

Adding More Pond Fish

A pond with mechanical and biological filtration and aquatic plants will better sustain fish than ponds without them. If you want more fish, you can add filtration and aeration so the pond can support them.

As for actually introducing new fish to your pond, you’ll want to follow the proper fish introduction procedure.

Overfilling Your Pond With Fish

Overfilling your pond with fish puts them at risk for health problems. The filters won’t be able to handle the amount of waste they produce, which lowers the water quality. If you want more fish, install larger or multiple filters. You’ll also need more pond and filter cleanings. You can learn how to clean pond filters so you can do it yourself!

Too many fish per gallon can overload the pond with ammonia, leading to sick fish. Crowding fish can make them uncomfortable because they won’t have any room to grow or swim! It would be like getting into a crowded elevator and needing to stretch your legs.

They also may breed and cause the pond to become overfull.

Get Your Pond Maintained The Premier Way

At Premier Ponds, we want to make sure that you have the best pond all year. That’s why we offer a variety of maintenance plans to choose from.

If you ever have any questions, our team is always available to answer them. So what are you waiting for? Give us a call or fill out our contact form today and let us show you how easy it is to have a beautiful pond in your backyard!