So, you’ve built your dream pond.

It’s just like you wanted it…

… stylish and bursting with lush aquatic plants.

There is only one thing missing.

Fish.

The two most popular pond fish options are goldfish and koi.

And it seems you can’t really decide between one and another.

The truth is that the choice isn’t that complicated, after all, once you weigh in all the factors.

What size is your pond?

Where is it located? If outdoors, what is the climate in your area?

Koi and goldfish might be cousins, but they have quite different requirements.

Answering the questions above, therefore, will give you a clear indication on which species is best for your system.

Now, let’s see when you should pick koi, or when you’d be better off with goldfish, and why.

Ornamental Koi Fish

To the untrained eye:

Koi and goldfish look surprisingly alike.

That’s because they both originated from the carp.

Ornamental Koi fish come from Japan and are nothing but a domesticated subspecies of the common carp.

They’re typically bred for their unique, colorful patterns that can add high-class flair to any pond.

To be honest…

… I’m fascinated when I watch them swim and interacting with one another.

Koi is a resilient species and can live up to 100 years, sometimes more.

Bigger than goldfish, they can reach lengths of up to 3 feet.

They thrive in ponds with high aeration, low waste, and stable water parameters.

Due to their possible larger size…

Some fishkeepers recommend no more than 4 koi fish in 1,000 gallons of water.

But not all koi will get this big for various reasons (source).

(Though most fishkeepers WANT a jumbo koi!)

Regarding water quality, koi thrive in slightly alkaline water with a pH between 7.5 and 8.

While the ideal water temperature should be between 65 and 75°F, they can handle anything between 35 and 85°F – a thing that makes them more suitable to keep in an outdoor pond in all seasons.

Ornamental Goldfish

Goldfish are a domesticated species of Asian carp.

They come in less colorful patterns than the koi and are smaller.

Today, you can choose from about 300 types of goldfish, either common or fancy.

In contrast to koi, goldfish come in a myriad of shapes and sizes…

… a thing that makes them a more versatile choice.

Their size varies from a couple of inches to about 1 foot long.

Are more suitable for smaller ponds and aquariums, and resist well at temperatures between 65 and 72°F.

However, the fancies typically don’t do too well in colder or hotter water.

They also have a shorter lifespan, usually of about 10 years max.

They do, however, tolerate a more acidic water pH, typically between 7.2 and 7.6.

While common/comet/shubunkin goldfish could survive outdoors in the winter, the fancy breeds would better be brought indoors when harsher weather arrives.

Depending on the breed, a large, full-grown goldfish typically requires between 10 and 30 gallons of water, which means you could keep between 30 and 100 exemplars into a 1,000-gallon pond.

They can also stay smaller though 😊

Related Post: How Big Can Goldfish Get?

Koi Vs. Goldfish: How to Choose?

Here are a few things to weigh in before deciding which breed to keep.

And remember:

Sometimes – you don’t have to choose!

Many people keep BOTH together.

The two can even reproduce with one another.

But there are marked differences between the two species.

1. Fish Pond Size and Type

Obviously, the first thing to consider is the size and type of your pond.

Do you have a fairly big, permanent outdoor arrangement?

Or a smaller, mobile pond?

Keeping in mind the water requirements of each breed…

… you should go for koi in the former case and goldfish in the latter.

In broad lines, you’ll have to provide 10 gallons of water per each 1 inch of fish if you want your pets to thrive.

Goldfish are also a more appropriate choice for a mobile pond, as you’ll be able to keep them indoors during winter.

2. Pond Depth and Climate

How deep is the pond also determines what breed of fish you can keep.

Koi are larger than goldfish, and they need a pond at least 3-foot deep.

If the winter in your area is harsh, the pond should be even deeper – around 4.5 feet if you don’t want all the water in the pond (and thus your fish) to freeze.

A shallow pond is more appropriate for goldfish…

… but keep in mind that most species need to be taken indoors during winter.

If you don’t have the possibility to move the pond or transfer goldfish into an aquarium…

… a surface heater might do the trick in a temperate climate.

However, this means higher pond maintenance costs.

3. Pond Maintenance

Both koi and goldfish have specific water quality requirements.

Regardless of the breed you keep, it’s ideal for your pond to have the following equipment:

  • Pond filter and pump to eliminate waste
  • UV sterilizer/clarifier to eliminate green algae and bacteria
  • Air pump to promote water aeration and oxygenation

You’ll also have to test the water quality constantly.

While all fish ponds have the same requirements…

… maintaining a larger pond requires bigger and more expensive equipment.

Your energy bill will also be more expensive due to higher energy consumption.

4. Feeding

No matter what aquatic plants you have or what fish you keep…

… your pond inhabitants will need food.

Both goldfish and koi carps should be fed 2 to 4 times a day.

However, goldfish are smaller; thus, they consume less food.

No matter what species you decide to go for:

Make sure to only feed your fish with high-quality food pellets formulated for the species and breed you want to keep.

5. Your Preference

Finally, consider the most simple (yet obvious) reason:

Your preference.

If you don’t like the fish you’re going to get, it won’t make the pond experience very enjoyable.

If you have a fish you aren’t fond of, you might not be as motivated to take good care of them either.

When it comes to goldfish vs. koi…

… Are you drawn to one over the other?

Then that one may be the best choice for you!

So, Which Fish?

Whether you should keep koi or goldfish is your call.

Weigh in all the factors above and decide which is the best option.

In broad lines, if your pond is 1,000 gallons or less…

… or if you’re a beginner…

… goldfish might be your best bet.

For a larger pond and experienced fish keepers, koi offer a wider variety of colors and higher resilience.

So, what do you say?

Would you populate your pond with koi or goldfish?

Tell me in a comment below; I’d love to hear from you.

And if you know someone else who struggles with the decision, share this article with them too.