40 Gallon Goldfish Tank – Some Important Notes You Should Know

You may see goldfish live together in a small tank at pet store. The size of the housing tank can significantly impact the health of goldfish. In order to maintain an oxygenated pool and remove harmful waste, goldfish need adequate swimming space, as well as proper water flow. When creating a healthy environment, you must plan your fish tank for the long term, determine how many goldfish you will keep, and choose the proper tank size accordingly. If you want to buy a 40 gallon goldfish tank, these things are what you should consider to make the best decision.

In A 40 Gallon Goldfish Tank Can Keep How Many Goldfish?

40 gallon goldfish tank can keep 2 common goldfish of medium size but up to 4 goldfish of small size. But, the number of fish you can house in a 40 gallon goldfish tank should be determined based on the species you choose. For example:

Fantail goldfish3
Single-tail goldfish1
Adult-sized Black Moor~1
  • For fancy goldfish (fantail goldfish), a minimum 20-gallon tank is required for a single one. Hence, a 40-gallon goldfish tank can house a maximum of 3 adult-sized fancy goldfish. Then, the tank needs to be each size up to roughly 10 gallons for each added individual.
  • For single-tail goldfish, such as common, comet, and shubunkin goldfish, a single individual needs at least a 30-gallon and 4-foot-long tank. Therefore, just one fish can fully occupy a 40-gallon tank. A pond would be ideal when you want to pair them with others.
  • For an adult-sized black moor (~ 10-12 inches), it would be a bit overstock for itself – 1 goldfish in a 40-gallon tank. Thus, you should provide it with a larger tank. So now you know how many goldfish are in a 40 gallons tank.
40 gallon goldfish tank
A fantail goldfish.

How Big Should Goldfish Tank Be?

Basically, a goldfish requires a tank with at least 20 gallons of water or about 3 feet/ 36 inches (91.44 cm) in length to freely swim. This is because goldfish are active swimmers and need plenty of space to move around. A larger tank size is always better, as it provides more space for the fish to swim and allows for a greater water volume, which can help maintain good water quality. It’s also better if you want to cohabit goldfish with other fish species.

It is also important to consider the shape of the tank when choosing a suitable size for a goldfish. Goldfish tanks should have a wider, rectangular shape rather than a tall or narrow design, as this allows for more horizontal swimming space. A tank with a proper filtration system and regular water changes will also help maintain good water quality and keep the goldfish healthy.

In addition to tank size, this tropical fish also require appropriate heater, freshwater, pH level, and a balanced diet to thrive. Overall, providing a suitable tank size and proper care can help ensure that your goldfish remains healthy and happy for many years to come. You also should put some decorations like live plants on surface area.

Importances Of Aquarium

While they are relatively easy to care for, it is important to understand the importance of providing them with a suitable environment. Having a well-maintained goldfish aquarium not only enhances the beauty of your home or office, but it also provides numerous benefits for the health and well-being of the fish.

So, let’s get to the details for each 40 gallon goldfish tank type. 

  • As the long and breeder tanks are less than 18 inches in height, the light intensity should be lower than the high tank types. LED or fluorescent light settings are recommended here.
  • Meanwhile, for the high tank with 20 inches in height, other high output types of fluorescent lighting such as T-5 high output or very high output (VHO) lighting systems, are more appropriate.

Filtration for 40 gallon goldfish tank

goldfish tank
Goldfish tank decorations.

As goldfish overgrow and produce a significant amount of ammonia, good filtration systems are very important for a 40 gallon goldfish tank.

Generally, the rule of setting filtration for each goldfish tank is that filtration should be at least 10x the tank capacity or 400gph. This means that a 40-gallon goldfish tank needs total filtration of at least 400 gallons. Though heavier filtration would be better, you should not ignore the importance of frequent big water changes to eliminate the mess of goldfish.

One of the popular and highly efficient methods for keeping water cycling and free from debris is biological filtration. A biological filter is all you need to do biological filtration. Each filter is sized differently to fit specific tanks, so make sure you get the right one. Biological filters require a carbon cartridge that needs to be changed once a month. Whisper and Top Fin are two companies that manufacture good biological filters.

40 Gallon Tank Types 

Differences between a standard 40-gallon and a 40-gallon breeder tank

You should be aware of the differences between breeder tanks and standard tanks when looking to buy a 40-gallon goldfish tank.

  • The difference in size: the breeder tank is generally wider than the standard tank. You can tell the difference by looking at the size tank listed above.
  • As the name implies, the breeder tank is designed for owners to easily view and handle their fish, contributing to successfully breeding their fish. The 40-gallon breeder tank has a more square base while the other 40-gallon goldfish tanks have a more rectangular base.
  • Both freshwater and saltwater fish can be housed in a breeder tank, as they can fit up to two adults. Moreover, rock formations or other decorations can be easily rearranged in the breeder tank than in the standard tank as it is wider. 

Which are the more appropriate tank types?

In fact, many owners choose breeder tank types to keep their goldfish though they do not have a certain intention of breeding their fish. Breeder tanks will provide more space for your goldfish to swim. So does the long tank type. 

Conversely, the high tank will display greater water depth, creating higher water pressure on the body and fins of your goldfish, which makes their swimming become more difficult. That’s why goldfish prefer thriving in the horizontal swimming space.

Hence, it depends on the space in your home and your preferences, but I suggest the long or breeder tank for goldfish.

The Best Way To Clean A 40 Gallon Goldfish Tank

Keeping a fish tank clean is not easy, but this task will effectively prevent cloudy water, algae growth, and harmful water-borne bacteria. For goldfish, you should change ~25% of the water weekly to keep their environment clean and healthy.

For cleaning a 40 gallon fish tank, you can use manual fish tank cleaning supplies. To remove algae, you can use an algae scraper or a magnetic algae cleaner, while to eliminate bacteria, applying a bleach solution is recommended.

It is important to make sure you get rid of chlorine and bleach residue on your tank by rinsing it off after cleaning.


Is a 40 gallon tank good for goldfish?

Depending on the type of goldfish, the tank size may vary. But in general, a 40 gallon goldfish tank is good for one to three goldfish.
It is recommended to prepare a tank with 50-60 gallons of water for two common goldfish or 4 small goldfish to healthily thrive.

Can 2 goldfish live in a 20 gallon tank?

The general rule for a goldfish tank is 1 gallon of water content/ 1 inch of fish size. This means it needs 10 gallons of water for a single small goldfish. Hence, you can keep two small-sized goldfish in a 20-gallon tank.

Can I keep goldfish in a 10 gallon tank?

Yes, you can keep one or two baby goldfish in a 10 gallon tank. It would help if you moved them to the larger tank as they’ve grown up.
You can read more about keeping goldfish in 10 gallon tanks here.


A goldfish requires an aquarium of at least 20 gallons (or a tank around three feet long). The additional 10 gallons of water are needed for each goldfish introduced to the tank after that.

Therefore, in a 40 gallon goldfish tank, 1 – 3 goldfish can live happily and healthily.

40 gallon goldfish breeder or long tanks should be better than high tank types. Filtration and frequent water changes are critical to keeping your goldfish’s living environment clean. For lighting settings for a 40 gallon goldfish tank, LED and compact fluorescent are good to consider.


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