How Many Fish Can You Put in One Tank? A Comprehensive Guide

How Many Fish Can You Put in One Tank_ A Comprehensive Guide

If you have ever experienced looking at a brightly colored fish tank, you know it can be a mesmerizing experience. Often, people are drawn to vibrant colors and a wide variety of plants and fish that can be included in the tank. If you are looking to start a fish tank, you need to make sure you do it the correct way to avoid unhealthy fish. 

How many fish can you put in one tank? The standard rule of thumb is one 1-inch fish per gallon of water.  However, this question is not one that is easy to answer because there are many variables when it comes to tank and fish size.  

In this article, you will learn the best way to determine the number of fish you can contain in one tank, as well as what happens if you try to put too many fish into a tank.

Considerations When Choosing the Number of Fish in a Tank

For a long time, the advice given when placing fish into a tank has been one 1-inch fish per gallon of water. While this may seem like a quick and easy answer, it is much more complicated.  

When placing fish into a tank, you need to consider the following things:

  • What is the body mass of the fish in the tank?
  • How much waste will be produced by fish?
  • How much swim space do the fish need to have?
  • What are the behavioral characteristics of the fish?
  • What are the specific species that you will be homing?
  • What will you be feeding them?

While these questions may seem a bit strange, it is important to answer each of these with fidelity when beginning a hobby of fish keeping.  Failure to consider the answers to these questions may lead to an overfull tank and unhealthy fish. 

What is the Body Mass of the Fish in The Tank?

While considering the length of the fish is essential when putting fish into a tank environment, it is also important to understand the mass of the fish’s body. 

Realistically you can have a rather small fish with a high body mass. This increased mass means the fish will need more room in the tank.

How Much Waste Will be Produced by the Fish?

Another critical and less pleasant thing to factor in is the amount of waste that will be produced by the fish in the tank. 

It should come as no surprise that some fish are quite dirty and produce a high volume of waste/fish poop each day.  So, while their body mass may be conducive to placing several in the tank, the amount of produced waste could cause a problem.

How Much Swim Space do the Fish Need?

According to Neil Monks of Tropical Fish Magazine, in his article Top Ten for The Ten Gallon Tank, when placing fish into a tank, you need to consider the amount of swim space they need.

Although it appears a fish may not need much exercise, you must remember they are natural swimmers and need to have adequate swimming space to remain healthy. 

What are the Behavioral Characteristics of the Fish?

There are many fish that are very docile and get along with all other species of fish. However, some are more aggressive. The aggression of the fish you place into a tank will determine how many as well as what type you can plan to home together.

Aggressive fish will fight to the death to control their territory, for Example Betta fish. You should not put your fish in a hostile environment.

What Specific Species Will You be Homing?

As a responsible fish owner, you need to be well-educated on the type of fish you plan to introduce to your tank. 

Take time to research the various species of fish and which species thrive together or in a fish tank environment.  Spending the time to educate yourself will save both money and heartache. 

What Will You be Feeding the Fish?

If you think a sprinkle or two of fish flakes will be enough to sustain your fish, you may be mistaken.  When you begin looking at exotic and less common fish, you also need to look at their dietary needs as well.

True, some fish will thrive with flakes, but others will need a more specific diet. As the owner, you need to make sure their needs are being met. 

What Problems Can Occur from Overcrowding?

What Problems Can Occur from Overcrowding Your Fish Tank With

After reading about all the considerations that need to be made when you begin adding fish to your tank, you may still be curious about what “really” happens when you add too many fish to the tank.

In this section, you will learn some of the difficulties that can occur from overcrowding your fish tank, such as:

  • Low oxygen level in the water
  • High ammonia levels in the water
  • High nitrate levels in the water
  • Fish aggression
  • Stunted growth

Part of you may be thinking that, if you accidentally add too many fish to your tank, you can just get rid of them.

But how do you plan to do that? Do you have a pet store that you can take the fish to? Are you planning to release them into the wild? How do you choose which fish to keep and get rid of? Remember, you committed yourself to care for them.

Low Oxygen Level in the Water

As you know, fish gather their oxygen for survival directly from the water.  If there are too many fish swimming and gathering oxygen from the water, it will be limited.

Imagine if you will, that you are scuba diving, and the instructor has given an oxygen tank to you and three other people to share.  At a certain point, the oxygen will no longer serve its purpose because it is being used by too many people. This is what can happen to your fish. 

How do You Know the Oxygen Level is Low?

Unfortunately, it is not always easy to notice the oxygen level is low in the water.  There aren’t any telltale signs in the water, such as color change, odor, or any other physical signal.

The best way you will know the oxygen is low is by the behavior of your fish.  When fish tank water gets low on oxygen, the behavior of your fish will likely rapidly change. 

Some of the behaviors you may notice are:

The fish will become lethargic – You must be aware of the ordinary behavior exhibited by your fish.  If you have a species that is already quite sedentary, this may not prove to be an accurate indicator.

The fish may appear to swim off-balance – Typically, fish swim in nice clean lines through the tank.  If you notice your fish flopping from side to side as they are moving or even laying on the bottom of the tank, there may be something wrong with the oxygen level.

The fish are losing their appetite – You may notice your fish are not eating like they used to.  While it is common for animals to go through periods of not being as hungry as usual, this would be considered atypical behavior if it appears rather suddenly. 

You see fast breathing – You may notice the fish are using their gills rather excessively or seem to be struggling to get air.  This can be an indicator they are struggling to get enough oxygen. 

The fish swim to the surface and open their mouths to get airIf you see the fish moving to the surface with wide mouths to get air, chances are the oxygen level is low.

The fish are congregating around the air filter – Usually, you won’t see your fish hanging out by the air filter for any length of time.  If it appears that this is the new fish hangout, you want to check your oxygen levels. 

As you can see, many indicators can tell you oxygen levels may be off. Unfortunately, they do not appear over a length of time; they appear suddenly. When you notice the behavior changes, you must act quickly to save your fish.  

How Do You Fix it?

After seeing how dangerous low oxygen can be to your fish, you may be wondering what you need to do to fix it quickly.  It is imperative to know, if this is a result of an over-crowded tank, the one correct way to ensure it doesn’t happen is to lower the number of fish in the tank.

If this isn’t an option, there are other methods, but you need to be keenly aware of the oxygen level to ensure it doesn’t cause a future problem. 

Maintain the correct number of fish for your tank size: This may seem like a no-brainer, but trust me, many people love fish and love a variety of fish.  Maintaining an appropriate number in the tank may be quite difficult for them.

Change the water: If the water has gotten dirty, it can impact the oxygen levels. By changing the water, you may help to remedy the problem.  If you have an overcrowded tank, you need to change the water frequently. 

Use an air filter: If you notice a problem with the oxygen consistently occurring, you may want to invest in an air filter if you don’t have one, or a stronger one if you do. The air filter will allow the water to circulate through the tank and will increase the available oxygen for the fish. 

Quality Air Filters

When you search for an air filter, you want to make sure it is going to fit the size tank you have.  It is better to err on the side of caution and provide a filter that is too big rather than too small.  You cannot over oxygenate your fish tank. 

Here are some quality air filters on Amazon you may want to check out:

By monitoring your fish tank carefully and having a knowledge of typical behavior for your fish, you will likely be able to avoid complications with low oxygen.  If you know that your tank is overcrowded and you can’t seem to part with any of the fish, please be vigilant about watching the oxygen level.

Can You Test the Oxygen Level?

Because low oxygen levels may not always be observable, there are methods you can use to test the oxygen level of the water. You can purchase dissolved oxygen test kits that will enable you to see the exact levels of oxygen in your fish tank.  

Here are some to consider:

High Ammonia Levels in the Water

High ammonia levels can be deadly to fish in a very short amount of time.  Many factors can contribute to high ammonia levels in a fish tank.

Here are the most common contributing factors that you need to be aware of:

Overfeeding: Believe it or not, fish don’t need a tremendous amount of food each day. If you happen to be overfeeding your fish, it is likely the food that is not being consumed is sinking to the bottom of the fish tank where it sits until it begins to decompose. Decomposition causes gases to be released, even in a watery environment. How mutch should you feed your fish, read here.  

Overcrowding: If your fish tank is overcrowded, the amount of waste will be greater, which leads to high levels of decomposition occurring at one time.  Remember, decomposition produces gas in the water that can become deadly to the fish shortly. 

Dirty Filters: If you have filters on your tank that have not been cleaned, they are not able to do their job of cleaning the water. This means when water flows over the filter, instead of releasing clean water, it may be carrying particles of decaying matter into the water, causing the gas situation to get worse.

Algae: You have surely seen algae coating either a pond or the side of a fish tank that is overdue for a cleaning. While this may not seem like a big deal, it can be life-threatening for the fish in the tank. Algae is an organic matter that will begin to decompose over time.  The decomposition will cause excess gas in the water as well.

Excess Waste: When you don’t clean your fish tank over a period, you are likely to have excess waste. This includes both fish urine and fish fecal matter. When the fish excrete waste, it typically sinks to the bottom of the tank, where it lays to begin the process of decomposition. When there is an excess amount of waste decomposing, a high concentration of gasses is released. 

How Do you Know the Ammonia Level is High?

There are specific test kits you can purchase to determine if the ammonia level is high in your fish tank.  Unfortunately, high ammonia levels are deadly to fish and can rapidly cause their decline, so you likely won’t see any indicators of high levels until it is too late.  Because of this, you must test the water frequently to ensure ammonia levels are kept at zero. 

What Happens When the Ammonia Level is High?

It is essential to know that any trace of ammonia above zero is dangerous to the fish in the tank.  You may be wondering what effects high ammonia has on fish. You should be warned, though, that the effects are incredibly unpleasant:

  • The ammonia will cause burns on the gills of the fish.
  • It can cause brain and organ damage.
  • It will increase the stress the fish is feeling.
  • It will cause the fish to lose their color. 

Remember, ammonia detection is highly unlikely without the aid of a testing kit.  Do yourself and your fish a favor and diligently test for ammonia.

Ammonia Testing Kits

Knowing that ammonia is a silent killer to fish makes it even more critical to have proper testing materials and procedures in place.  

Here are some quality at home ammonia tests you may want to consider purchasing:

High Nitrate Levels in The Water

If you have ammonia in your fish tank, there is a strong possibility that you have nitrites as well. While nitrates are a byproduct of ammonia, they are equally harmful and must be dealt with accordingly. Nitrates are formed when ammonia is present in the water. Remember, ammonia is created mostly because of waste that is decomposing in the tank.  

How Do You Know the Nitrate Level is High?

The only real way to determine if you have nitrates in your fish tank is to test for it regularly. Like ammonia levels, nitrates are not easily seen and can kill your fish rapidly without intervention. When the nitrates get too high in the tank, it will cause the fish to suffocate because the nitrates hinder the amount of oxygen that can be carried by the blood.  

Testing for Nitrates

Knowing how important it is to test for nitrites, here are some tests you may want to check out:

Fish Aggression

When you try to cram too many fish into a tank, you will likely notice some fish aggression beginning.  While it may seem inconsequential, fish aggression can be a big problem.  When fish are aggressive to one another, they will try to injure each other, and ultimately, if left alone, they will try to kill each other.  

What Causes Fish Aggression?

There aren’t many fish that are genuinely aggressive because of personality per se.  In her article Aqua scaping Tips to Prevent Fish Aggression, Kate Barrington shares some reasons fish may act aggressively. 

Fighting for territory – Typically, fish like to have their territory for mating, having a family, and just having their own space.  Often, they will gravitate to lush plants or rocky areas you may have built up in the fish tank.

Mating – Fish will show aggression toward other fish when they are in the process of “courting” a potential female mate.  If the fish feels like others are infringing on their territory, they will become aggressive.

Food – If a fish feels that they are unable to get enough food for survival, they may also become aggressive.

True, some fish exhibit these characteristics frequently, but they will likely be triggered and enhanced when a fish is placed in an overcrowded situation.  They are going to feel the need to fight to ensure their basic needs are being met.

How Can You Stop Fish Aggression?

If you are noticing aggressive fish, you likely want to put an end to it before injuries or death occurs. The single best thing you can do is make sure the fish have enough swimming space.  This may mean upgrading the size of your fish tank or decreasing the number of fish in the environment.  

Stunted Growth

Another problem that occurs when too many fish are placed in a tank is stunted growth. You may have heard the adage that a fish will only grow as big as the container they are put in. While this may make sense, it isn’t truly the reason a fish stops growing. provides a comprehensive article detailing the science behind the stunted growth of fish in captivity as well as the longevity of fish with stunted growth in the article Stunted Growth Means Stunted Lives.

Fish are equipped to release a hormone that helps determine the space they are in as well as the other fish in their area. This hormone is responsible for the progress of their growth. When a fish is put into an overcrowded situation in a too-small tank, it can cause stress to interfere with the production of this hormone.

When the hormone production is interfered with, the fish will halt its growth.  Nutrition can also play a role in the size of the fish. The healthier the fish is, the larger it will tend to grow. 

What Size Fish Tanks Are There?

How Many Fish Can You Put in One Tank - What Size Fish Tanks Are

There are many different sizes of fish tanks that can be purchased. Fish tanks are so popular that you can even have them custom made. When you begin looking for fish tanks, you will notice they are classified by three different size options.

  • Regular 
  • Long 
  • Show

Regular Fish Tanks

This is likely the type of fish tank you have or will have. These are the typical tanks that you find in locations like Wal Mart, Target, Pet Co, Pet Smart, and other pet stores. While some can be quite large, they are usually on the smaller size and are quite manageable for a home setting. On the other hand, some enthusiasts declare, that the bigger fish tanks are easier to maintain

Here are some examples of regular tanks and fish tank setups available on Amazon, you may be interested in:

Regular tanks can be anywhere from 1 gallon up to 100 gallons or more. The size you choose depends on the type of fish you plan to keep. There is not such a thing, as Too Big Fish Tank.

Long Fish Tanks

Long tanks offer a beautiful viewing tank. They are not quite as deep as regular tanks, which provides ample area for people to admire the fish you choose to put into them. Think of a long fish tank as something that you could easily put on a bookshelf. Because they are so specific, you will likely need to special order a long fish tank. It is doubtful that you will find one at a general pet store.

Show Fish Tanks

If you have ever walked into a restaurant, zoo, museum, or any other building that has a gigantic fish tank as a focal point, you have seen a show fish tank. Just as the name indicates, a show fish tank is just for that purpose. It is to show off the fish enclosed in the tank.

Usually, these fish tanks are equipped to hold large numbers of fish to display. These tanks need to be specially made to your specifications. 

Final Thoughts

Deciding to keep fish as a hobby can be a significant undertaking. If you are planning to combine different species of fish, you need to be willing to learn about them and spend the necessary time observing them to ensure they stay strong and healthy.  

It is also vital that you understand the volume of fish your tank can hold. Trying to add too many fish to a tank can have detrimental effects. As a pet owner, it is your responsibility to make sure the fish have plenty of room as well as a quality environment to live in.

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How Many Fish Can You Put in One Tank? A Comprehensive Guide