Find out How Many Goldfish You Can Keep in A 20-Gallon Tank

Many people choose to keep goldfish as pets because they are low-maintenance and easy to care for. However, there are still some things to consider before bringing home your new pet fish. One crucial factor is the size of your tank. A 20-gallon tank is suitable for a few goldfish, but you’ll need to research to figure out the maximum number of fish your tank can hold.

It is recommended that one gallon of water per inch of fish is an excellent rule to follow when keeping a live fish tank. A 20-gallon aquarium can hold 20 tiny goldfish or two or three adults. It’s critical for your goldfish’s health and well-being that you provide them with enough room to swim freely.

Continue reading to find out how many goldfish can be kept safely in a 20-gallon tank and how to avoid overpopulation.

20 Gallon Tank: How Many Goldfish Can You Keep in It?

Goldfish prefer larger tanks. They need lots of swimming room and are typically best kept in tanks that hold at least 10-gallons. They can grow to be a foot long, so make sure there is plenty of room for that. Considering the size the goldfish can get as an adult, the ideal number is two or three goldfish for a 20-gallon tank.

Another vital thing to consider for your goldfish tank is that it must be extremely clean. Goldfish quickly pick up diseases from dirty water, making it easier to contract diseases from other fish.

If you continue to clean the tank and change its water frequently, your goldfish will usually thrive. Remember, the bigger the tank, the easier it is to keep the water values stable, especially if you have many live plants.

fancy goldfish

How Often Should You Change the Water in A 20-Gallon Goldfish Tank?

Changing the water in your goldfish tank is essential because it is good for the overall health of your fish and because the water in the fish tank evaporates over time, depleting the water and nutrients available to your fish.

The amount of evaporation depends on the temperature of your tank, the quality of your water, and the amount of light that the tank receives. Some goldfish tanks will lose up to a quarter of their water in just one year.

As far as how often you should change the goldfish tank’s water, the answer is that it depends on how clean the water is. If you have a lot of fish waste in the water, you should consider changing it every couple of days, especially when your goldfish tank is in use. Otherwise, goldfish tanks tend to need less-frequent maintenance, especially if you take care to regularly skim the water, extract the waste, then dispose of it.

You will want to ensure that you clean your tank regularly, even when there are no fish. Over time, detritus builds up in the water, and you don’t want it accumulating to the point that it limits the oxygen levels. Thus, it’s best to check your goldfish tank at least every two weeks to ensure clean water, remove any solid waste, and check the pH and ammonia levels to ensure no fluctuations.

Regular maintenance is essential for your goldfish tank because it ensures a healthy environment for the fish and because it creates less work for you.

SeaClear 20 gal Acrylic Aquarium Combo Set, 24 by 13 by 16", Clear

What Size Fish Tank Should I Buy for My Goldfish?

Purchasing a tank for your goldfish does not have to be complicated. Some people assume that goldfish can only grow to the size of their tanks, but others insist that this is not the case. Professionals feel it’s critical to provide your goldfish with adequate room to swim freely without continually colliding with other fish in the tank.

Although goldfish are freshwater fish and do not require a heater, your tank temperature should be between 50 and 70°F. Goldfish can also produce a lot of waste, and therefore a good filter is essential.

If at all possible, avoid tall or novelty tanks. For goldfish, rectangular aquariums are typically the best option.

Make careful to account for materials like pebbles, plants, and other unique decorations placed in the tank. This will take up significant room, and when you would have 20 fish in a 20-gallon tank, once the decorations are in place, they may only have 10-15 gallons of area.

goldfish below water surface

How Do I Know if My Goldfish Tank Is Overcrowded?

For the health and well-being of your goldfish, sticking to the ratio of 1 inch of fish per gallon of water is an excellent rule to follow.

However, if you suspect your fish tank is overcrowded, there are a few essential indicators to keep an eye out for. These indicators will tell you if you have too many goldfish in your aquarium.

Does Your Tank Have Low Quantities of Oxygen?

Examining the oxygen levels in your fish tank is one of the best ways to tell if it’s overcrowded. This isn’t easy to establish and necessitates a thorough examination of your fish’s behavior and tendencies. Low oxygen levels might indicate too many fish in a small area.

Look for indicators of low oxygen levels such as a lack of swimming and feeding, heavy breathing, or gasping for air near the surface.

Does Your Tank Have High Amounts of Ammonia?

Another symptom of having too many fish in one tank is high ammonia levels. The presence of ammonia should be treated carefully because this substance is very harmful to fish and can result in death.

Because of the increasing quantities of waste goldfish create, fish aquariums are prone to high ammonia levels. It’s produced by waste breakdown, and even low concentrations can cause significant difficulties.

The easiest way to correct this issue is to ensure you have a good water filter. However, if your tank is congested, the ammonia will not be quickly neutralized by the good bacteria, causing problems for your goldfish.

Does Your Take Have High Levels of Nitrate?

Your goldfish tank’s nitrogen and ammonia levels are linked because ammonia is produced when helpful bacteria break down. Although nitrate isn’t a big deal at low to medium concentrations, it might create significant difficulties for your goldfish if you have a high concentration level in your tank.

Too many fish frequently cause high nitrate levels in a single tank. Bacterial and fungal infections might result as a result of this.

Making sure you have a proper filtration system and regularly changing the water in the tank is the best approach to counteract this. Adding living plants that feed on nitrates is also an intelligent way to deal with this issue.

Are Your Goldfish Showing Signs of Aggression?

As you may expect, an overcrowded goldfish aquarium can lead to behavioral issues. When there are too many goldfish in one area, they may become violent toward one another, and it can also cause certain fish to become highly territorial.

Allowing your goldfish to wander about freely in the tank will result in a calmer and more tranquil life. Due to a shortage of space, fights might break out, resulting in injuries such as fin nipping. As a result, needless stress and infections may occur.

Has Your Goldfish’s Growth Been Slowed?

Some aquarists feel that an overcrowded tank may impede goldfish development. Fish are thought to secrete growth-suppressing hormones to adapt to their surroundings. If they do not have the resources to do so, this inhibits their growth.

This is a contentious theory. Some people believe this is factual, while others feel it is just fiction.

5 Reasons to Keep Your Goldfish in A Large Tank.

  • Goldfish have a bulkier form than other fish, and as a result, they require more oxygen than other fish. When a goldfish matures, it expands four times its original size, necessitating the use of extra oxygen. As a result, you should give the fish plenty of room; otherwise, it will be panting for air near the bottom of the tank.
  • A goldfish’s body is designed to develop to a length of at least 6-8 inches. Unfortunately, most pet goldfish never get the opportunity to do so. Pet owners often overlook the value of a big tank. Your goldfish will be able to grow properly if you maintain it in a tank with the appropriate room. If you don’t give it adequate space, it will get stunted and unhealthy. The goldfish’s lifespan will be decreased as a result of this.
  • Owners of goldfish who maintain them in a tiny tank frequently change the water to keep it clean. On the other hand, goldfish is a species that is constantly shocked by variations in water temperature. When done in a tiny tank, it disturbs the goldfish much more. Unlike huge tanks, when you replace the water in a small container, it cools or heats up faster. There is more water in huge tanks, which slows the temperature change process. To avoid your goldfish being shocked, use large containers instead of small dishes.
  • Nobody enjoys being left alone for the entire day, and goldfish are no exception. They are lonely, just like us. It is impossible to maintain a single goldfish happy in a single dish. To keep your goldfish happy, you’ll need a big tank that can hold at least two fish.
  • No matter how gorgeous the fish are, a full water tank never looks good. It’s also harmful since many fish in a tiny container would quickly contaminate the water. Furthermore, goldfish dislike being kept in an overcrowded aquarium. When the aquarium is overcrowded with fish, they become violent. If the goldfish begin fighting with one another, they may develop wounds and fungal diseases.

Final Verdict

Many people choose to keep goldfish as pets because they are low – maintenance and easy to care for. However, there are still some things to consider before bringing home your new pet fish, including how many goldfish can be kept safely in a 20-gallon tank and how to avoid overpopulation.

Goldfish are not as easy to maintain as most people think, and proper fish tank conditions are essential. In this article, we have reviewed how to keep your goldfish in a large tank, know if your tank is overcrowded, and what to do if it is.


My name is Jaap, and I am a passionate shrimp and fish enthusiast. I rediscovered my love for this hobby with my son and want to share our knowledge and tips with you on this aquarium blog. We cover topics such as setting up and maintaining an aquarium, feeding and breeding your shrimp and fish, and creating the perfect environment for them to thrive.

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