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

How Many Fish Will Fit In A Pond Per Gallon?

Find Out How Many Fish You Can Comfortably Fit Into Your Maryland, DC, Or Northern Virginia Koi Pond

Would you want 20 dogs running around your house? It would get cramped really fast. The same can be said for how many fish in a pond you want. In this case, it’s better to have less fish and them be happy and healthy than risk putting in more and making them…uncomfortable. No pond owner wants this! Read on to learn more about how many fish per gallon you should have.

What To Consider When Adding Fish

“Like sardines in a can” isn’t a description pond owners want for their ponds because of the drawbacks.

Too many fish per gallon can overload the pond with ammonia and make your fish sick. 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. When deciding on the amount of fish your pond can hold, keep these factors in mind:

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 proper fish introduction procedure.

Now let’s get into the rules of adding fish to a pond!

Number Of Fish Per Gallon

Here are some basic rules determining how many fish your pond can sustain:

  • Rule 1: one 6″ fish for every hundred gallons of water
  • Rule 2: 1″ of fish per 10 gallons of water
  • Rule 3: 1” of fish for every square foot of surface area (24” deep pond)

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, so if you want more fish, install larger or multiple filters. You’ll also need more regular cleaning and maintenance on your pond and filters. To see these rules in action, scroll on down!

Testing The Fish Per Gallon Rules

Let’s test these rules using an average size pond that’s 10’ x 15’ with an average depth of 18” (1.5 feet) and is 24″ at the deepest point. We can also assume it’s a liner pond with rock and gravel.

Calculating the water volume: 10′(length) x 15′(width) x 1.5′(av. 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 volume). So the true volume of our 10’ x 15’ pond is about 1,266 gallons.

  • Rule 1: 12.7 (6”) fish or 76.2 inches of fish
  • Rule 2: 126.6 inches of fish or 21 (6”) fish
  • Rule 3: 112.5 inches of fish (using the length x width x .75)

How Many Fish In A Pond- Other Factors


Don’t buy enough fish to fill your pond. Leave fish some breathing room so they can grow to their full size without crowding the pond. Size doesn’t include the tail.


Fish are like underwater rabbits- they have a lot of babies. They normally spawn during late spring/early summer when the water is warmer. To get around this, many people remove the smaller koi and goldfish during cleanings.

Fish Per Gallon, Fish Per Inch

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

See what else is part of a pond ecosystem >>