Pond Fish Stocking Calculator

Enter the pond volume and choose goldfish or koi. You get a careful number of fish to keep and the upper limit, both counted on the fish when grown.

Units
The water really in the pond, worked out from the average depth.
Fish
Common and comet goldfish can reach 12 in (30 cm) in a pond. Plan for the grown fish, not the size you buy.
A careful number to keep5 goldfish200 US gal (757 L) each
Upper limit10 goldfish100 US gal (379 L) each

A pond of 1,000 US gal (3,785 L) carries about 5 goldfish of 10 in (25.4 cm) with room to spare, and 10 at the most. That is counted on the grown fish: 50 to 100 inches of fish in all.

Careful rule
Half an inch of fish for every 10 US gal: 50 in (127 cm) of fish in 1,000 gal
Upper limit
One inch of fish for every 10 US gal: 100 in (254 cm) of fish
  • These are numbers for a pond with a working filter and pump. With no filter, keep far fewer fish.
  • Pond fish breed. Leave room, and stock below the careful number if you can.

What to work out next

Pond volumeWork out the volume firstPond pump sizeFlow for a stocked pondCommon goldfishSize, water and company

How many fish can a pond hold?

Pond keepers count fish by length, because a big fish makes far more waste than a small one. Two rules are in common use:

Rule Water for the fish Use it as
1 inch of fish for every 10 US gallons 10 gal (38 L) per inch The upper limit
Half an inch of fish for every 10 US gallons 20 gal (76 L) per inch A careful number, with room for growth

The second is the more careful figure that Aquascape publishes, because pond fish grow and breed. The calculator shows both.

Number of fish = pond gallons ÷ gallons per inch ÷ length of a grown fish in inches

Worked example. A 1,000 gallon pond and goldfish that will grow to 8 inches.

  • Upper limit: 1,000 ÷ 10 = 100 inches of fish. 100 ÷ 8 = 12 goldfish
  • Careful number: 1,000 ÷ 20 = 50 inches of fish. 50 ÷ 8 = 6 goldfish

Koi need more

Koi grow much larger than goldfish: 24 inches (60 cm) is common. A fish vet who works with koi puts the floor at 250 US gallons (946 L) for each koi, whatever size it is today. The calculator never allows more koi than that.

For a 1,000 gallon pond that means 4 koi at the most, and 2 if you want room to spare.

Count the grown fish

The most common mistake is to count the fish at the size they are sold. A 4 inch koi from the shop is the same animal as the 24 inch koi it becomes. Stock for the grown fish, and the pond stays healthy as they grow.

What these numbers assume

  • A working filter and pump. With no filter, a pond carries far fewer fish.
  • Enough oxygen, from a waterfall, fountain or air pump, above all in hot weather.
  • Regular care: partial water changes and a clean filter.

Stocking rules are guides, not laws. A pond with strong filtration can carry more; a pond in full sun with little water movement carries less. Add fish a few at a time and test the water as you go.

Common mistakes

  • Counting fish at shop size. Count them at the size they will reach.
  • Treating koi like goldfish. A koi needs many times the water.
  • Using the volume of the hole, not the water. Work the volume out from the average depth.
  • Filling the pond to its limit on day one. The filter needs weeks to catch up with each new batch of fish.
  • Forgetting that pond fish breed. Next year there may be many more.

Questions and answers

How many goldfish can I keep in my pond?

As an upper limit, one inch of grown fish for every 10 US gallons: about 12 goldfish of 8 inches in a 1,000 gallon pond. Half that number leaves room for growth and breeding.

How many koi can I have in a 1,000 gallon pond?

Four at the most, and two is the careful number. Each koi needs at least 250 US gallons, because koi commonly grow to 24 inches.

How many gallons does one koi need?

At least 250 US gallons (946 liters) for each koi. More is better, as koi keep growing for years.

Can goldfish and koi live in the same pond?

Yes, they are often kept together. Add up the grown lengths of all the fish and keep the total inside the limit for your pond.

Do I need a filter for pond fish?

For any more than a very few fish, yes. These stocking numbers assume a working filter and pump; without them the pond carries far fewer fish.

Numbers for standard tanks

Have a standard tank? Its size, weight, heater and stocking are already worked out.

Sources

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