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How Many Goldfish in a 40 Gallon Tank? (Quick Answers)

Aquarium tanks are home to pet fish, but the difference is they can not go out of it. They have to spend their entire lives in it. So, it’s essential to get the correct number of fish according to the tank size. Goldfish are one of the go-to options for new aquarium hobbyists. 

So, how big a tank do they need? How many goldfish can live in a 40 gallon tank happily? This article will explain the tank requirements and other essential tips for keeping goldfish.

How many goldfish in a 40 gallon tank?

Keeping two goldfish is suitable for a 40 gallon tank, and one should not keep more than three goldfish in it. Goldfish need ample space to swim, feed and stay healthy freely. A 40 gallon tank fulfills those requirements of 2 goldfish. A bigger aquarium is required for large fancy goldfish species.

There is a famous saying that one-gallon water is needed for every inch of fish. That works in many different situations but not for the goldfish. Goldfish can grow about 10 inches, and if you follow the 1-inch-1-gallon rule, you can keep at least four goldfish. 

But actually, it’s not suitable for keeping four goldfish in a 40 gallon tank. Keeping two goldfish is perfect for a 40 gallon tank. Let me explain why.

That rule is suitable for small and baby fish, but goldfish grow larger so fast that you will have to keep changing tanks if you buy small goldfish and keep them in a small tank. 

So, while purchasing the goldfish, you should get a large permanent tank considering that they will grow fast.

That is the most basic reason for keeping two goldfish in a 40 gallon tank. But there are many more.

Goldfish create a lot of waste too fast. That means they make the water dirty within a few days. So a strong filter is essential for them. But no matter how strong the filter is, if you keep them in a small tank, the wastes will harm the tank’s water quality.

In tanks, fish are mostly safe. We keep them at the proper temperature, provide food regularly, and they also don’t need to hide from predators like wild fish. But the biggest enemy of pet fish is their waste. 

The waste has some harmful chemicals that drain the life out and make them unhappy.

Most new fish keepers lose their fish as they are not careful about the water quality. In a large tank, the ammonia buildup process is slow as there is more water. So, you will not have to go through frequent water changes if you have a larger tank. 

For example, if you have two goldfish in a 30-gallon tank, you will have to change the water every five days. But if you have a 40 gallon tank, you will get a seven day gap between the water changes.

There is another issue with frequent water changes. Fish like to stay in a similar environment, and frequent temperature change is harmful. Water replacement changes the water temperature, which is unsuitable for the goldfish.

Tank shape is also essential. Goldfish are slow swimmers, and they need some space to swim around. That’s why wider tanks are better for the goldfish than the deeper ones. We will dive deeper into the topic, but first, let’s find out the tank requirements for different goldfish breeds.

Fancy goldfish:

Fancy goldfish usually grow bigger than common goldfish or have some genetic modification. Such as, red cap oranda goldfish need more fresh water as it is harder for them to breathe for their fancy cap. 

That’s why it’s recommended to keep them in an extra-large tank rather than a regular goldfish tank. You can keep two fancy goldfish in a 40 gallon tank, but 50 or 60-gallon tanks will be ideal.

Fantail goldfish:

Fantail goldfish are a popular type of fancy goldfish, and two of them can live in a 40 gallon tank.

Comet goldfish:

Comet goldfish can grow up to 12-14 inches, so they need a larger tank as their permanent home. Two comet goldfish can live in a 40 gallon tank, but larger is better.

Can 2 goldfish live in a 40 gallon tank?

Two goldfish can live in a 40 gallon tank. A well-shaped 40 gallon tank will fulfill all their living requirements. Common breeds of goldfish can spend their whole life healthily in a tank like that if you can maintain all their necessities. 

However, larger species will outgrow their tank size, and they may need a larger tank after some years.

Tank size is the first thing you should take care of. But there are many things you will have to learn about fish keeping. You will have to maintain water health, a healthy microbial community, nitrogen cycle and other essential things to keep your fish healthy for their whole life.

Many new fish keepers don’t do the first step right, and many aquarium shops mislead new fish keepers with wrong information about the fish keeping. So, make sure you do research before listening to the seller.

How big do goldfish get in a 40 gallon tank?

If you keep two goldfish in a 40 gallon tank and do everything right, they can grow 7-12 inches, depending on how well you take care of them.

There is a myth that goldfish do not grow large in a small tank. But that’s not the actual issue. The problem is that it’s hard to maintain good water quality in a small tank, and the fish can be stunned by that. Or else they can be stunned if you do not feed them properly.

But goldfish can grow large in a small tank if you do everything right. However, that does not mean that goldfish in a small tank will be happy.

3 factors that determine how many goldfish can live in a 40 gallon tank

Here are the factors determining how many goldfish can live in a gallon tank.


40 gallon tanks are suitable for two common goldfish. You should not keep goldfish alone, so you need a larger tank for two fancy goldfish.

Size of fish:

If you have small-sized goldfish, you can keep 5-6 on a 40 gallon tank, but not for very long. But for a permanent tank 40, gallon tanks are suitable for two goldfish.


You can keep three common goldfish in a 40 gallon tank if you have strong filtration.

How to take care of goldfish in a 40 gallon tank?

Common goldfish are pretty hardy, but there are some sensitive breeds of goldfish on the market. So if you are a new fish keeper, you should start with common goldfish breeds. Here are some tips to keep goldies happy in a 40 gallon tank.

Don’t change the whole water:

Never change the whole water. Change 40-50 percent of the water every seven days, depending on the water condition.

Don’t wash the filter with soap:

Never wash the filter with soap. The filter inhabits beneficial bacterias that keep the nitrogen cycle running.

Don’t overfeed:

Never overfeed your goldfish. Goldfish can become fat fast, and that’s unhealthy for them.

Remove the fish waste regularly:

Remove the visible fish waste with a siphon regularly. It will help to remove the ammonia content in the water.

Don’t keep small gravel:

Don’t keep small gravel as an aquarium substrate. Waste and ammonia will get stuck between the stones, which is hard to remove.

What fish can I put in 40 gallon tank with goldfish?

A 40 gallon tank is too small to keep other tank mates with goldfish. If you wish to keep some, you can keep a pair of snails without any issue. However, if you can manage space, there are a lot of fish you can put in a tank with goldfish. 

Tankmates must have similar temperature requirements, and the other fish can not be too small that goldfish can eat. Here are some of the excellent tank mates of goldfish.


Platies can be kept with goldfish without any issue as they are also pretty easy to maintain. Platies are bigger than goldfish’s mouths, so goldfish can not eat them.

Rosy barbs:

Rosy barbs can also be good tankmates of goldfish, but they can grow up to 6 inches, so you need a bigger tank.

Zebra dinos:

Dinos are tiny, and they require the same temperature as the goldfish.

Hillstream Loach:

Hillstream Loaches are small in size, and they are good bottom feeders. They will eat the food waste by the goldfish.

Mystery Snail:

Mystery Snails can also be a good option for a goldfish tank. They will eat the algae and clean the tank.

Final Thoughts

A 40 gallon tank is the minimum size for keeping goldfish permanently, and you can keep two goldfish in it. It’s better to provide larger tanks for fancy goldfish as they can become larger. The goldfish can enjoy a healthy life if you are attentive to keeping the water quality right.