what happens if your fish tank is too small?

Can Fish Die If The Tank Is Too Small?

It’s not easy to calculate the right size tank for your fish. However, as an owner of pet fish, you must choose a tank that allows fish enough space to grow and swim. If you don’t, they can become ill and depressed.

Fish can die if they’re housed in tanks that are too small. They’re at risk of stunted growth, aggressive tankmates, stress, ammonia poisoning, and reduced oxygen levels. Fish develop several bacterial diseases as a result of low water quality. Overcrowding is a problem in small tanks, as is choosing fish that grow large.

When choosing your tank size, consider what fish you’re planning to have inside it. That way, you can determine the fish’s exact space requirements and ensure a safe, comfortable environment.

What Happens If Your Fish Tank Is Too Small?

Cramming too many fish into a tank causes a range of problems. Fish that don’t have enough room to swim or grow suffer from a shortened lifespan due to severe health and behavioral issues resulting from their environment.

If in doubt, always opt for a larger tank, as it offers your fish more room to live. Similarly, it’s easier to fill a tank with more fish than to reduce the population. If your fish tank is too small, expect to experience the following problems:

Stunted Growth

Does a small fish tank stunt fish growth? Many owners believe that fish grow to the size of their tank. While this isn’t strictly true, a small tank limits a fish’s ability to grow properly.

“Stunted” means that fish aren’t able to fulfill their species’ size potential. They also don’t grow at the right speed. Most fish stunting occurs without the owner noticing, so it’s difficult to tell when fish are experiencing stunted growth without seeking professional advice.  

A fish’s growth is determined by somatotropin. As described by Creative Diagnostics, somatotropin is a hormone that plays a vital role in growth control. It tells fish when to gain mass and is stimulated by exercise.

When tanks don’t have enough space for fish to exercise, hormone production decreases. Similarly, in an overcrowded tank, the feedback loop of hormones stops fish from growing. It also delays reproduction and weakens the immune system. As well as the right size tank, fish need the following to grow:

Fatty Liver Disease

Also known as hepatic lipidosis, fatty liver disease is common in adult fish whose growth rate has slowed. As already mentioned, a small tank contributes to this.

Fatty liver disease is when excess fat coats the liver, preventing the organ from functioning correctly. When fish are affected, they’ll refuse to eat as their digestive system starts to shut down. If left untreated, it causes death.


Small tanks make fish aggressive because there’s not enough space for them to swim. Some fish species are territorial and will fight each other for their own piece of the tank, especially if there are few plants and decorations to claim. When fish fight each other, you’ll notice the following:

  • Tears around the mouth and fins
  • Missing scales
  • Damaged eyes
  • Dislocated jaws

Some fish fare worse than others, especially if they’re smaller and more submissive than their tankmates. Dominant fish will bully them in order to gain territory.

In turn, small tanks don’t have enough space for plants, meaning there are fewer hiding spaces for fish to hide in to protect themselves.

are small fish tanks bad?

Poor Water Quality

Fish tank water quality problems are common when the aquarium is too small. Small tanks prevent the nitrogen cycle from working efficiently, causing: 


Cramming fish into small tanks results in increased levels of ammonia. Ammonia (NH3) is a nitrogenous waste. It’s present in all tanks, but beneficial bacteria convert it into nitrite.

A second bacteria convert nitrite into nitrate, which is naturally cleared out of the tank by water changes. At elevated levels, ammonia poisons fish and causes them to become unwell. In the worst cases, they die.

Ammonia is caused by fish excrement and urine. When the tank’s the right size for the number of fish, this isn’t a problem. But then too many fish are crammed into a small aquarium, large amounts of fish waste allow ammonia to infiltrate the tank. Symptoms of ammonia poisoning include:

  • Fish gasping at the water’s surface
  • Lethargy
  • Lack of appetite
  • Red or cloudy eyes
  • Redness around gills
  • Poor scale quality
  • Foul odor

As a result, you must choose the right size tank for the number of fish you have to prevent ammonia from rising to dangerous levels.

Not Enough Oxygen

Fish need oxygen to survive, as it helps keep the respiratory process working efficiently. Fish tanks also need oxygen to break down waste and remove harmful chemicals from the water.

Similarly, live aquarium plants require oxygen to survive. Without it, they decompose and raise ammonia levels.

Tanks with a larger surface area have more oxygen, as the gas exchange is more efficient. As a result, tanks shouldn’t just be large enough for your fish, but they should be wide enough to provide them with all the oxygen they need.


According to Aquatic Veterinary Services, chronic stress is caused when fish aren’t able to swim freely.

Poor water quality, over-crowing, and aggressive fish caused by a small tank are all leading causes of stress. Stress is also one of the main reasons for most health and behavioral conditions in fish. Signs of stress include:

Mild stress is something your fish can recover from, but if a small tank isn’t swapped for a larger one quickly enough, they can get ill from the stress. They may even die.

How Do I Know If My Fish Tank Is Too Small?

Many new owners make the mistake of choosing a starter tank, which is often too small for their fish’s requirements. As a result, their fish quickly outgrow their living space.

It’s not always easy to know how big your aquarium should be, but these are the most noticeable signs that it’s too small:

Lack of Swimming Space

If you notice that your fish aren’t swimming as much as you’d expect them to, it’s likely because there’s not enough room in the tank for them.

If this is the case, fish will swim sideways or remain at the bottom of the tank. Similarly, they may float at the water’s surface.

As already mentioned, if there isn’t enough room in the tank for fish to swim comfortably, they’ll begin to chase each other out of frustration. Fish that hide more often are trying to get out of the way of other fish as there’s not enough space for all of them.

As a result, increased or unusual activity is the sign that something’s amiss in the tank. As a rule of thumb, if fish can swim from one end of the tank to another in a single motion, the tank is too small, and you’ll need to upgrade to a larger tank.

Loss of Appetite

Stressed or unhappy fish will stop eating. This is a common reaction to poor tank conditions. Both of these things are the direct result of a small tank, which is a serious problem. When your fish stops eating, they’re are alerting you that something’s wrong. In this case, their tank’s too small.

Feeding times can also be stressful in a small tank. This is because fish don’t feel safe enough to eat when they’re too close to other territorial fish. Stress is also the leading cause of appetite loss.

Not only is appetite loss a problem, but uneaten food festers and rots at the bottom of the tank, causing harmful ammonia levels to rise and decreasing the water’s overall quality.


Diseases form as a result of stress or improper water conditions. Common health conditions and diseases that are the result of a small tank include:

  • Ick, which is caused by a parasite
  • Dropsy, a bacterial infection of the kidneys
  • Fin rot, a bacterial infection that causes the tail and fins to fray
  • Pop eye, which is a bacterial infection of the eye
  • Clamped fin, which is the result of an infection or parasite
  • Red or white sores, usually as the result of fish fighting

Fungal and bacterial infections are also common when the tank’s too small and not set up correctly. This is why it’s crucial to get a tank that’s the right size for the number of fish you have.

What Size Fish Tank Do I Need?

All fish need different tank requirements, so, unfortunately, there’s not a one-size-fits-all solution. The size of the tank you’ll need depends on how many fish you have. You’ll also need to do some research to work out whether your chosen fish prefer wider or taller tanks.

To work out the right size tank for your fish, use one of these three popular measuring methods:

One Inch Per Gallon

The most popular way to calculate the appropriate tank size is with the one inch per gallon rule. This measurement assumes that a tank can hold one inch of fish per gallon of water, and uses the fish’s length as the primary measurement.

While this method is a handy rule of thumb, there’s still a risk that the tank could end up being too small. This is because many owners forget to account for plants and ornaments.

It also doesn’t take into account the fish’s height. As a result, measure the plants and any decorations and account for them as part of the tank to ensure enough swimming space.

Another useful calculation is to work out your fish’s height and add 10 gallons of water for every fish.

One Inch Per Two Gallons

Not all owners like the one inch per gallon rule because it can lead to overstocking if it’s not calculated correctly. The one inch per two gallons rule is a better method to allow for growing fish, reducing the risk of stunted growth.

The concept is the same as the one inch per one-gallon rule, but it allows fish more swimming space. It also means the water quality stays better for longer, requiring fewer changes.

When using this calculation, measure the size of your plants and ornaments, as this leaves plenty of space for fish to swim and grow.

Aquarium Floor And Surface Area

The most accurate way to find the right tank size is by calculating the aquarium’s floor space and surface area. This method provides plenty of room for both plants and fish and allows for a wider tank floor. This can help prevent aggression and low water quality within the aquarium.

Our calculation is based on fish requiring 12 square inches of surface space, but you can adjust this to suit the size of your fish. To get your calculation, follow these steps:

  1. Measure the length and width of your tank and multiply the two to find out the surface area.
  2. Divide the surface area by 12. This will give you the total number of inches of fish your tank has space for.
  3. Divide the answer you’ve gotten by 12, which is the height of the fish.

Why Is My Small Fish Tank Cloudy?

Cloudy water can be baffling – and not to mention worrying for fish owners. Few fish tanks have crystal clear water, but cloudiness indicates a problem within the aquarium.

The best way to prevent your tank’s water from going cloudy is to provide a large enough tank to accommodate your fish and plants comfortably. Reasons for murky water are linked to a tank that’s too small and include:


The most common cause of cloudy water is a “bacteria bloom.” When ammonia and nitrites build up within the tank, aerobic bacteria establish themselves. As it floats through the water, a cloudy appearance appears.

An increase in ammonia causes a bacteria bloom. This is usually the result of uneaten, decomposing food, which provides nutrients for the bacteria to grow. The tank’s filtration system also can’t clean the bacteria quickly enough, causing it to multiply.


Low water quality as the result of overcrowding is another leading cause of cloudy water. If there’s not enough room for all the fish you have inside the tank, the water’s quality degrades rapidly. Fish create excess waste that fills the tank up too quickly, causing the filtration system to work too hard.

Because a tank is a closed ecosystem with no water coming in, the water quality deteriorates quickly in a small tank, making it cloudy. Similarly, if water changes aren’t carried out frequently enough, the water will get dirtier.

Small tanks require more water changes than larger tanks, which are often neglected by naïve fish owners.


A sudden spike in fish waste causes algae to rapidly multiply, covering the tank’s plants and glass while causing the water to turn murky and opaque.

Fish waste acts as a fertilizer for algae. Too much in a small space makes algae grow out of control. Algae also require the same nutrients as bacteria to grow, so a cramped tank provides the optimum conditions for an algae bloom.

As well as too many fish inside a tank that’s too small, too much light and overfeeding causes algae overgrowth.

Are Small Fish Tanks Bad?

Fish tanks that are too small are far worse than tanks that are too big. Large tanks allow space to grow, whereas small aquariums offer cramped conditions that no fish are able to thrive in.

Another problem with small tanks is that problems escalate quickly. The small volume of water that fish have to live in experience sudden water chemistry and temperature changes, which can be fatal.

You must carry out water changes more regularly to keep the tank clean of harmful chemicals. If this step is neglected, ammonia and nitrite could rise without warning. 

You might also be wondering; is it cruel to keep fish in small tanks? The simple answer is yes. While you’re acting with good intentions, you’re creating a stressful and harmful environment for your fish.

why is my small fish tank cloudy?

Easiest Fish To Keep Alive In A Small Tank

When looking for fish that are suitable for a small tank, you need to choose fish that won’t grow too large. For example, goldfish are seen as a good choice, but they grow surprisingly big, meaning they outgrow many starter tanks. As a result, consider the following small fish for your tank.

Bloodfin Tetra

Also known as the true bloodfin, bloodfin tetra is a freshwater fish that’s easy to care for, making it popular with beginners. It grows to a manageable two inches long, with a lifespan of 5-7 years.

The bloodfin tetra fish has a silver body with a bright green tail and fins. The minimum tank size for these fish is between 10-20 gallons, depending on how many you have. 

Bloodfin tetra prefers living in schools of around 5-7 fish. They swim in groups in the upper and middle sections of the tank and love lush, live vegetation that replicates their natural South American habitat.


The guppy fish is also known as the millionfish and rainbow fish. It’s one of the most popular tropical freshwater fish species, adding color to the tank. They’re also easy to look after.

Male guppies range from 0.6-1.4 inches in length, while females are slightly larger, reaching 1.2-2.4 inches. Guppies do best in groups of six with a ratio of two females for each male.

Because they’re so small, guppies don’t need much space to live in. However, the recommended tank size provides four gallons of water for a group of three guppies. When adding more to the tank, opt for:

  • 4 gallons of water for 3 guppies
  • 6 gallons of water for 6 guppies
  • 9 gallons of water for 9 guppies


Platy is another freshwater fish species. They’re peaceful and easy to care for, making them great for beginner fish owners.

Males reach around 1.5 inches in length, while females grow up to 2.5 inches. Both males and females live for approximately 3-5 years.

While platies are small, they’re active, so they need enough space to swim around in. Platies thrive in a 10-20 gallon aquarium. They can also tolerate a wide pH range of between 6.8-8.5, so they’re adaptable to their conditions.

Platies are native to Central and North America and do well in a tank that contains plenty of plants. Platies aren’t schooling fish, so they shouldn’t be kept in groups of more than five. The ideal ratio is one male to three females.

White Cloud Mountain Minnow

White cloud mountain minnows are small, colorful fish. They grow to around 1.5 inches and live between 3-5 years.

White cloud mountain minors enjoy living in schools of up to 12 fish. They don’t like being alone. As a result, they lose their color and spend most of their life hiding out of stress.

The minnow is undemanding when it comes to their tank conditions, but they don’t do well in temperatures about 72 F.


Also known as zebra danio, zebrafish are a favorite freshwater fish with beginner fish owners because of how easy they are to care for. Zebrafish are defined by their black and white stripes, which gives them their name.

As adults, zebrafish grow to two inches. While they only need a 10-gallon tank, they’re prolific breeders, so they need to be monitored to ensure they don’t multiply too quickly. They thrive in schools and also make peaceful community fish alongside other species.

Small tanks are bad news if you’re planning to fill them with plenty of fish and plants. Not only can they cause a range of physical health problems, but they cause depression and anxiety. Therefore, you must choose a tank that fits your fish’s size requirements and allows them to grow and swim comfortably.