How Many Goldfish Can Be Kept in a 40-Gallon Tank?

Alison Page

Alison Page


how many goldfish in a 40 gallon tank

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The humble goldfish has been one of the most popular aquatic pets in the world for many years. There are around 200 varieties of these beautiful fish to choose from. They’re relatively low-maintenance, and they can live in an aquarium or an outdoor fish pond.

However, there’s one question that’s asked by many beginners who are thinking of adding a goldfish to their collection of pets – “how many goldfish can I keep in my aquarium?”

Goldfish need to live in groups to be happy, and a 40-gallon setup is a suitable tank size for between three and four fancy goldfish. However, many factors come into play when it comes to working out how many fish your tank can hold.

Read this guide to learn more about how many goldfish you can safely keep in a 40-gallon aquarium.

What Factors Determine How Many Goldfish Can Live in a Tank?

Three main factors determine how many goldfish you can keep in your aquarium:

Goldfish Size

The goldfish you buy in your local pet store are juveniles that can grow from 8 inches to over 12 inches long, depending on the variety. Therefore, it’s essential that you choose the right tank size for them.

The first and most important consideration when deciding how many goldfish can live in a tank is the size of the fish.

As mentioned earlier, there are over 200 varieties of goldfish, and they come in many different shapes and sizes.

For example, most types of fancy goldfish grow to between six and eight inches long, whereas slim-bodied goldfish, such as Comets, Common goldfish, and Shubunkins, can easily grow to over a foot long.

So, you can see that a fish tank is not the best environment in which to keep slim-bodied varieties of goldfish, and these goldfish always do much better when living in a spacious outdoor fish pond.

As a general rule of thumb, you need to allow at least 10 gallons of water per goldfish. However, remember that your substrate, decorations, and plants all take up space in the tank, so you might not have as much water volume to play with as you think.

Water Quality

Although goldfish are pretty hardy creatures, they are also extremely messy, producing a lot of waste. For that reason, a goldfish tank must have an efficient, powerful filtration system running 24/7 two keep the water clean and safe for the fish.

Your filter system should be able to turn over four times the entire water volume of the aquarium every single hour. That means if you have a 40-gallon size tank, the filter must be able to handle at least 160 gallons per hour.

In addition, you’ll need to carry out frequent water changes to keep the water conditions clean. Note that in larger tanks, the increased water volume means a reduced maintenance workload for you and makes it easier to keep your goldfish healthy.

Goldfish Species

The most suitable type of goldfish to live in an aquarium is the fancy goldfish. There are many varieties of fancy goldfish to choose from, all of which have different personalities and swimming abilities.

For example, Oranda goldfish don’t have a dorsal fin, making them quite slow and clumsy, whereas Fantail goldfish grow slightly larger and are much more competent swimmers.

In both cases, you will need to allow plenty of swimming space in the setup to avoid injuries that might be sustained if the fish collide with decorations or each other.

As mentioned above, a fish tank is not suitable for slim-bodied goldfish that must live outside in a pond.

Often, small fancy goldfish are all displayed together in pet shops, even though there may be several varieties in the tank. So, be sure to ask the assistant in the fish store to confirm what type of goldfish you are buying to avoid confusion.

How Many Goldfish Can Live in a 40-Gallon Tank?

how many goldfish in a 40 gallon tank

So, bearing in mind all the above factors, I recommend that you keep no more than three or four fancy goldfish in a 40-gallon fish tank.

That larger fish tank size should provide the fish with ample space for swimming and socializing without overcrowding them, leaving you with happy and healthy fish.

When it comes to the kinds of tanks you should use, shallow tanks are better than deep tanks for fancy goldfish. A shallow tank provides plenty of surface area for better oxygenation and makes it easier for the fish to swim to the surface to feed.

What Are the Dangers of Overstocking Your Fish Tank?

A common mistake beginners make in this hobby is overstocking their tanks. Overstocking is very bad news for you and your pet fish for the following reasons:

Poor Water Quality

The more fish you keep in a single tank, the more waste they produce, and that waste will very quickly pollute your water. The resulting decline in water quality will lead to stress for the fish, compromising their immune system and leaving them more open to attack by parasites and diseases.

You can mitigate that to some extent by performing weekly partial water changes and running an efficient filter system. However, it’s much better not to overstock your tank capacity in the first place.

Oxygen Deprivation

Goldfish are known to be oxygen-hungry creatures, and overstocking your tank can lead to a decline in the available dissolved oxygen in the water. In severe cases, that can lead to suffocation for your poor goldfish.

Stressed Out!

A lack of swimming space and constant bumping and barging with their tank mates is highly stressful for the fish. Stress impacts the fish’s immune system, leaving them vulnerable to diseases and making them more likely to be attacked by parasites such as flukes and Ich.

Although Goldfish aren’t generally aggressive toward each other or other species, overcrowding can lead to competition for food, sometimes resulting in squabbles and minor injuries. In addition, slower individuals might go without food altogether, potentially leading to malnutrition and even starvation in extreme cases.

Final Thoughts

Did you enjoy our guide explaining how many goldfish you should keep in a 40-gallon tank? If you did, please take a moment to hit the share button above!

I recommend keeping no more than three or four fancy goldfish in a 40-gallon tank. These fish can grow to reach between six and eight inches long and require at least 10 gallons of water per fish to be comfortable and avoid overcrowding and the poor water quality that can cause.

Do you keep your goldfish in a tank or an outdoor pond? How many goldfish do you have? Tell us in the comments box below!

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