40 Gallon Goldfish Tank – 5 Important Notes You Should Know

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, there are 5 notes you should consider to make the best decision.

A 40 Gallon Goldfish Tank Can Keep How Many Goldfish?

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. Therefore, theoretically, 40 gallon goldfish tank can keep 2 goldfish of medium size but up to 4 goldfish of small size.

 But, the number of goldfish you can house in a 40 gallon goldfish tank should be determined based on the species you choose. For example:

  • For fancy 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 roughly 10 gallons for each added individual. 
  • For the single-tail goldfish, such as common, comet, and shubunkin goldfish, a single individual needs at least a 30-gallon and 4-feet-long tank. Therefore, just one fish can fully occupy a 40-gallon tank. A pond would be more ideal when you want to pair them with others.
  • For an adult-sized black moor (~ 10-12 inches), it would be a bit cramped when being in a 40 gallon goldfish tank by itself. Thus, you should provide it with a larger tank.

Essential 40 Gallon Goldfish Tank Dimensions

40-gallon tank types

40 gallon goldfish tank can be divided into breeder tanks and standard tanks (40-gallon long tank and 40-gallon high tank).

So, the size of the two tank types is different. 

  • A 40-gallon breeder tank is about 36 inches, 18 inches, and 16 inches in length, width, and height, respectively.
  • A 40-gallon long tank is 48 inches, 12 inches, and 16 inches in length, width, and height, respectively.
  • A 40-gallon high tank is 36 inches, 13 inches, and 20 inches in length, width, and height, respectively.
Breeder and standard goldfish tank types
Breeder and standard goldfish tank types

As you can see, the 40 gallon breeder tank and long tank can give more swimming space to your goldfish as they are longer and wider than the high tank types. Before getting your hand on a real 40 gallon goldfish tank, you may want to understand the difference between the breeder and long 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.

Lighting For 40 Gallon Goldfish Tank

When purchasing a lighting system for your 40 gallon goldfish tank, you should consider the tank height. If your tank is 18-inch high, you can have a wide-range option. Meanwhile, for higher tanks (between 18 and 24 inches), you will need a light with a bit stronger intensity. When the tank is more than 24-inch high, a high-intensity light setting is required.

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.
Simple 40 gallon goldfish tank with lighting system
Simple 40 gallon goldfish tank with lighting system

Filtration For 40 Gallon Goldfish Tank

As goldfish overgrow and produce a significant amount of ammonia, 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 for 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.

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.

Video: 40 Gallon Goldfish Tank – Fancy Fantail Goldfish Planted Aquarium


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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