Fish Count Tank Sizes (Aquarium Calculator)

As a fish keeper of many years, I have spent quite some time learning and understanding fish tank sizes for different species of fresh and saltwater fish. In this article, I’ll share with you different fish count take sizes.

Fish Count Tank Sizes – Calculator

There is a general rule of thumb for keeping fish of various types and the amount of fish you can have in a tank depending on size. For this reason, we decided to come up with a calculator to determine how many fish you can have in a tank. There are many people in the fish community with different opinions but from my own experience and experience of others who I trust we cam up with these numbers.

This is not an actual fish in a fish tank calculator but a general rule of thumb to reference when deciding how many fish to put in your tank.

Consider this, would you want to be stuck in a hot elevator with 10 people for a long time? I doubt it and the same goes for fish of any breed. Now, the smaller the fish breed the more you can get away with.

Also keep in mind that some fish don’t pair well together and for that reason some fish should not be in the same tank. Be sure to watch any new fish additions to be sure they are not being bullied.

Neon Tetra

One of the most popular fish to have in your aquarium is the Neon Tetra. These tropical freshwater fish are South American natives along the Amazon River basin.

They grow to roughly 1.5 inches when it comes to length.

This means that they do not need much room for them to thrive and be happy.

Every Neon Tetra that you put in your aquarium needs one gallon of water. Nonetheless, since Neon Tetras are among the schooling fish, it means that they tend to be in their happiest mood when more of their kind are around.

For this reason, have more than one of them in your tank. Putting at least five of them will go miles to maximize their well-being.

Having established that, a five-gallon tank would work as the minimal tank requirement. That is one gallon for every fish you add.

Note: As we have already established, Neon Tetras are among the schooling fish. If you decide to keep them, make sure you buy a big aquarium. Choose one that holds not less than 10 gallons or 38 liters of water.

Doing so goes miles in making sure that you provide the necessary swimming and hiding space for your Neon Tetras.

It is prudent to estimate roughly 10 gallons for every 24 of these fish that you intend to keep.

Beta Fish

Betta fish is another popular species that you can keep as a pet in your aquarium. In most cases, pet store owners keep them in tiny containers.

Because of this reason, many people think that they normally do not need big tanks. This is not the case.

This fish species can grow up to 3 inches, meaning they will need plenty of room for them to not only hide but also swim.

Many people prefer to keep this species of fish alone. There is no harm in doing that.

A 2.5-gallon tank can be an incredible or healthy size as the bare least for one Betta fish. If you intend to put more, it is prudent that you have only one male.

Males tend to be aggressive towards each other.

Increase the amount of water you put into your tank by 2.5 gallons for every Betta you intend to get.


In most instances, when you think of a goldfish, you picture it inside a tiny bowl, right? No! Contrary to the opinion of many, small containers such as a tiny bowl is not great homes for goldfish.

Do you know that goldfish need big tanks? Yes, if you plan to keep one as your pet, your tank must hold at least 10 gallons for every goldfish.

Another misconception many people have is that goldfish grow into their tanks. This is not even close to the truth.

Keeping goldfish inside small-sized tank stunts their growth. It is quite unhealthy for them.

While in captivity, they can grow to an average of 5 inches especially if there is ample room for them to grow.

However, in the wild, they can reach up to 14 inches. If yours happens not to be as big as five inches, chances are high that you probably require a bigger tank.

For every goldfish you have, you are supposed to increase your tank by 10 gallons.

For instance, if you have five goldfish in your tank, you need to have at least 50 gallons of water.


Guppies are very similar to Neon Tetras in terms of the kind of housing they require. In terms of size, they are small.

They can only grow up to around 2 inches when it comes to length. This means that individually, they do not need more than one gallon of water to thrive and be happy.

Therefore, a one-gallon tank would work fine.

Just like Neon Tetras, these are also schooling fish. They tend to feel much secure, safer, and happier when with a large group to accompany them when it comes to swimming.

Experts recommend that you have not less than five guppies in your aquarium for them to be able to school. What does this mean?

It simply means that you need to have a tank whose capacity is not less than five gallons.

Note: The bigger the tank you can provide for your fish the better.

Zebra Danios

Another fish species that are popular and easy to keep is Zebra Danios. These small fish can grow to roughly 2 inches when they grow fully.

Because of their love for swimming, the bigger the tank you keep them, the happier and more comfortable they will be.

However, while that is the case, it is important to note that at the minimum, you should put them in a tank that can handle 2.5 gallons of water for every fish.

Another great highlight of this type of fish that you need to know is that this fish species is schooling. Therefore, it is prudent that you put 5 fish in one tank.

This translates to a water tank that is 12.5 gallons. If you are unable to get a 12.5-gallon tank, just know that a bigger size is always better compared to a smaller one.


If you are looking for an easy to keep and popular saltwater fish, then you should settle for this one. You can easily find them in almost any saltwater tank.

These species of fish turned out to be highly popular and a darling of many after the film Finding Nemo.

They can grow to roughly 3 inches. For them to be comfortable, it is prudent to keep them in a tank of roughly 30 gallons.

One thing you need to know about this fish species is the fact that they are into hiding. They live in sea anemones.

Therefore, if you have plans to get sea anemone for a clownfish, it is prudent to compensate it with a bigger tank of roughly 30 gallons.

This makes it a total of roughly 60 gallons.

Six Line Wrasse

The Six Line Wrasse is our next fish to look at. They also make very good pets especially if you are into saltwater fish.

They boast the same housing needs as their clownfish counterparts. Six Line Wrasse grows to be roughly 3 inches in terms of length.

Each one of them needs a 30-gallon tank. Of great importance to note about these is the fact that taking care of them is effortless.

You can keep them with other species of saltwater fish without having to be worried about them having issues. They do not require any special needs or treatments.

Green Chromis

Green Chromis is another highly popular saltwater fish. These are peaceful and beautiful fish.

They grow to roughly 3.5 inches. They can be mates with other saltwater fish. Therefore, you do not have to be worried about pairing them with other fish.

These fish species are akin to the six-line wrasse and clownfish. You should keep them inside a 30-gallon tank.

Another highlight of this fish that you need to know is the fact that schooling them is not necessary. However, if you have plans of getting more than one of them, you must put at least six of them or more to ensure you maintain the pecking order.

A number that is less than this will result in the Alpha fish eliminating the smaller ones. So, what does this mean?

It simply means that if you have plans of schooling this fish species, you would have to have a tank with a capacity of 180 gallons of water.

Yellow Tang

Yellow Tang is another fish worth looking at in terms of its tank size requirements. The fact that it is a darling of many explains why you can find it in many saltwater aquariums and tanks.

This fish species grows to roughly 7.9 inches. The Yellow Tang requires at least 75 gallons of water to be healthy, happy, and survive.

Auriga Butterflyfish

If you have been searching for a beautiful and calm saltwater fish to keep as your pet, you should consider the Auriga Butterflyfish. In most instances, it is prudent to strictly keep this fish species in big aquariums due to their tendency to grow to approximately 9 inches long.

For them to thrive and be happy, keep them in a tank with the ability to handle 125 gallons of water.

This explains why most fish owners are not into keeping Auriga Butterflyfish due to their big tank requirements. These tend to be more ideal for bigger aquarium displays in hotels or offices.

You do not need to school them. However, they prefer having places to hide.

Determining The Safe Holding Capacity Of Your Aquarium (Methods to Ascertain Fish Count & Take Sizes)

Aquarists employ different methods to determine the appropriate number of fish to keep in their aquarium. The two most popular methods are:

  • One inch per gallon
  • Surface area

One Inch Per Gallon

This is an excellent golden rule if you are starting in fish keeping. You can also use this method if you have small community fish in your aquaria such as platies, rainbowfish, bettas, and tetras.

Nonetheless, it is inappropriate for larger fish species such as tinfoil, plectostomus, channel catfish, and Oscars. The same is true for messier species such as African cichlids, koi, piranhas, and Oscars.

You need to avoid using this method if you are keeping coldwater fish such as bass, gars, gragonfish, and goldfish.

Coldwater, messier, and larger fish species all require more available resources. This means that you must have only one inch of fish for every 3 or more water gallons.

Note: Some fish varieties tend to be more adamant and demanding regarding space and territories that they are in.

This rule for determining fish count for your aquarium works as a rough estimate. Consequently, it leaves so much room for error.

For starters, the rule does not consider modern filtration systems, and whether one has proper lighting or live plants. Additionally, it does not consider the kind of fish you keep.

Just like humans, fish are of different shapes and sizes. Stocking a 10-gallon tank with 10 inches of Zebra Danios which is slender shaped is different from stocking it 10 inches of Goldfish that tend to be full-bodied.

Of great importance to note is the fact that larger-bodied fish tend to create more waste compared to their smaller counterparts. For this reason, they will need more volume of water.

Another important aspect that you need to keep in mind is that fish need adequate space to swim. Some need more space than others.

While the numbers might look great on paper, in actual sense, the tank might be too tiny for fish to make movements normally.

This is especially true for the active fish species and schooling variety. You need to keep the schooling species of fish in multiples.

This means that you will need more space. By adding one or two schooling fish, particularly in small-sized tanks, you will only stress and shorten the lifespan of your fish.

You also need to appreciate the fact that when you first bring your fish home, it is normally not grown fully. That adorable tiny catfish that is one inch long today can turn out to be a half or one-foot size within months.

When doing your calculations about tank stocking, it is prudent that you consider the size of an adult fish.

Sadly, many fish keepers are usually clueless about how large or old their fish will grow to be. The prudent thing to do before you purchase a fish is to extensively research your prospective fish to ascertain its actual adult size.

The good thing is most pet stores usually have signage on tanks featuring information regarding the fish. Such information includes things such as the actual adult size of the fish and which other fish species can be ideal tank mates.

Another error that is associated with this method is assuming the aquarium size is equal to the number of water gallons it holds. What does this mean?

For example, a 10-gallon tank filled with plants, rocks, and gravel coupled with an assortment of decorations cannot hold 10 gallons of water. In reality, water volume is usually 10 – 15% less compared to the actual tank’s size.

In final words, although the One-Inch Per Gallon rule is and has always been a reasonable yardstick, it brings with it its flaws. To be on the safe side, follow everything we have highlighted in this section.

For example, when dealing with bigger fish species, always apply the rule – one-inch per two water gallons. It is prudent to be safer than sorry. That is for sure.

Surface Area

This is the other method you can use to establish the number of fish you can keep in your aquarium. What you need to know is that the bigger the water surface area, the more the exchange of oxygen to support more fish.

As a result, the water surface area has a direct impact on the number of fish you can keep in your aquarium. A thin and tall tank might hold a similar number of gallons as a wide and short tank, yet they bring with them completely different surface areas.

When we use the surface area rule, it is important to note that the shape of the tank is usually taken into consideration. You can calculate the surface area by multiplying the length by the width of a tank.

Under this rule, you can stock your tank with fish that measure one inch for every 12 square inches in terms of surface area.

Nonetheless, this calculation has its flaws just like its one-inch rule counterpart. For example, designing of this rule was such that it assumed that fish tend to be slender-bodied.

This is normally not the case, at least in many instances. If you keep wide-bodied fish inside the tank, you need to change the calculation to one-inch of fish for every 20 square inches in terms of surface area.

Both rules are not perfect.

The man advantage of this rule is the fact that it considers unusually shaped aquariums.

Factors That Determine the Maximum Aquarium/Tank Population

The real holding capacity of an aquarium which is also known as the maximum tank population varies based on several factors. The factors are all related to biological filters, oxygen distribution, and dilution of water products inside the water.

Let us look at some of the factors that determine the population of fish in an aquarium:


This is one of the most important factors that affect the fish population in an aquarium. Simply put, what is the amount of waste that biological elements of a filtration system that is being utilized process.

Various brands, models, and filter types can process different fish waste densities. Power filters, even those that feature a rotating drum filter cannot handle the volume of decomposing organic matter and fish waste that well maintained under gravel filters can manage to.

Under gravel, filters can only handle a part of what a great trickle or wet-dry filter can handle. It is worth noting that canister filters tend to handle more water compared to a power filter.

However, it is not as effective as a trickle filter. Of great importance to note is that the available filters boast different capacities depending on the model and make of the filter.

Water Dissipation

The ability of water inside the tank to dissipate fish waste as it is released is another factor that informs the population of fish in an aquarium. Tanks with larger volumes tend to keep their water cleaner, even disregarding the filtration system in place.

When you have a bigger tank and install an excellent filter system, you are certain that your fish will enjoy good health.

Oxygen Availability

The amount of dissolved oxygen in water is another essential factor that determines the number of fish you can keep in an aquarium. The greater or higher the surface area, the higher or greater the tank’s holding capacity.

Any filter, be it a canister or power filter increases a tank’s theoretical load. However, for this to happen, the filters must not only excellent flow rate but also need to be sufficient for your tank.

Trickle filters offer huge increases when it comes to the surface area. They get the solution from atmospheric gases inside the water closer to the proportions contained in the room’s air.

Biological filters need oxygen for optimal functioning. For this reason, your biological filter tends to be more efficient inside a well-aerated tank.

A filter might take a fair portion of oxygen that is dissolved in water, thus not leaving any additional for extra fish.

Nonetheless, even having several filters inside the tank won’t raise the tank’s holding capacity.

Aquarium Maintenance

When considering your prospective tank’s capacity, it is important to consider the level of maintenance that will be needed for your tank.

Great maintenance such as filter cartridge replacement, and frequent, small water changes will not raise the tank’s capacity but failing to do it will decrease its capacity for sure.

Fish Size

In most if not all instances, all the fish that are available inside a pet store or shop are babies.

At best, they can be juveniles. Frequently, they will need some growing to do after you get them home. You need to account for this in the space available not only for your current tank but also for planned or anticipated upgrades to the larger tanks.

With most fish, crucial information regarding their adult size is usually available on the labels of the tanks inside the pet store.

You should also consider the size of the adult fish versa vie the tank’s dimensions. Online and offline, you can find information regarding the minimum and a maximum number of fish that you should keep in your aquarium.

Effects Of Aquarium Overpopulation

While almost every fish keeper desires to have an array of fish species on his or her aquarium, having too many can be troublesome. Many people are clueless when it comes to controlling the number of fish to add to their aquarium.

Overcrowding your aquarium will result in the following:

Lower Oxygen Levels

One of the main indicators that your aquarium might be overcrowded is low levels of oxygen. Establishing this is difficult.

So how do you go about it?

A sure-fire way is to observe how the fish behave. Watch the following patterns to establish whether your fish are suffering from low oxygen levels:

  1. One of the ways that you can tell your fish lack oxygen is if the fish do not eat or swim.
  2. As the oxygen levels deep further, your fish will begin to breathe heavily. The good thing is that this is visible. Their grill movements turn out to be more rapid as they search for extra oxygen.
  3. Another indication of low oxygen level is the constant grasping of fish at the water surface as they search for more air. This is a sure-fire indication that your aquarium has low levels of oxygen.

Of course you must always remember that certain fish species normally gasp at the water surface as they search for air. It is normal for them. Therefore, you should not be worried or concerned when such a thing happens.

An excellent of such a fish species is the betta fish.

Nonetheless, if it is repetitive, chances are very high that the reduced oxygen issues are present in your aquarium. What is more, is the fact that not every fish is affected in similar proportions by the deficit in oxygen levels in the environment.

High Nitrite and Ammonia Levels

Of great importance to note is the fact that ammonia, coupled with its decomposing by-product nitrite, are highly harmful or toxic to your fish.

They are a product of waste decomposition. Even in small amounts, these two can result in severe issues and even the death of your fish. An excellent way of dealing with such an issue is by efficiently and properly filtering your water.

Ensure that the aquarium is properly cycled. Moreover, a nitrification process ought to be completed before adding fish in the tank.

In most cases, if your aquarium is overcrowded, the important or beneficial bacteria will not manage to neutralize nitrites and ammonia as fast as necessary.

This can go miles to cause plenty of issues to the fish.

High Levels of Nitrate

Another issue that is related to aquarium overcrowding is high levels of nitrates. What you need to understand is that nitrate is nitrite’s by-product.

It comes about when important or beneficial bacteria decompose ammonia.

While it might be toxic to fish in tiny to medium concentrations, a high concentration of nitrates can cause serious issues to your fish. High nitrates concentration gradually affects your fish negatively in countless ways.

This comes because of compromised immunity that emanates from high levels of nitrates.

An effective way of dealing with such an issue is by changing your aquarium’s water frequently. This is magical in terms of removing high levels of nitrates and replacing it with water that does not contain nitrates.

Alternatively, you can add live plants to feed off the nitrates. This is another way of efficiently dealing with the issue.


Aggression is another issue that is associated with an overcrowded aquarium. Overstocked tanks often result in your fish turning out to be overly territorial.

If you are not careful, such an issue can result in plenty of aggression. This is the last thing you want to happen in a closed environment like your aquarium.

Injuries and fin-nipping can result in infections and stress. Both can result in serious issues for your fish.

Note: Overstocking or overcrowding your aquarium can be a great thing in some instances. You can use it as a strategy to reduce the tension in the tank.

Some aquarists usually utilize overstocking as one of the counter-aggressive methods when it comes to keeping African cichlids.

Stunted Growth

An overcrowded aquarium can lead to stunted growth of your fish. A good number of aquarists think that fish growth and development can be based on the tank’s size.

Others believe that their tank’s size is informed by the fact that their fish do not or do have ample space for thriving and to be happy.

While in most instances this is not normally the case, it can be a factor when it comes to fish growth. The truth is that no consensus has been attained on this matter.

Some people believe that fish usually release growth-reducing hormones that can hinder their growth in such a way that they adapt to their environment.

Unhealthy water conditions and stress are the other two factors that can inform the reason fish do not reach their full potential when inside an overcrowded environment.

There is no doubt that this makes sense. Logically, fish can’t be happy and healthy or reach their maximum potential in terms of size and growth without a friendly environment to do so.

Common Issues Related To Small Aquariums

You should not have to grapple with many issues when dealing with a big aquarium. However, this is not the case when owning an aquarium that is too small.

When it comes to aquarium sizes, the bigger the tank or aquarium, the happier and healthier your fish will be.

Putting your fish in an overly small aquarium brings with it an array of issues.

Here are the top three problems of a small-sized aquarium:

Reduced Lifespan

One of the major issues that affect fish because of being placed in a small aquarium is reduced lifespan. You reduce their life if you put them inside a small enclosure.

In addition to stunt their growth, placing yours in an undersized tank or aquarium hugely reduces their lifespan.

Scientific research suggests that this happens because of two main reasons:

  • The number one reason is stress. Fish kept inside a small tank tends to be highly stressed. The stress tends to put an excessive amount of tension on the heart. In addition to that, it causes the fish to overwork.
  • Another reason is internal organ failure. As we have already established, when you place your fish inside a small tank, it will cause stunted growth. This simply means that the internal organs of the fish do not have ample space to allow effortless and healthy growth. In the end, this will result in internal organ failure.

Improved Aggression

Another issue associated with a small aquarium is enhanced aggression. This means that fish inside a small aquarium grapple with behavioral issues.

Small aquariums cause behavior implications like anxiety, fear, and stress. Experts opine that the biggest behavioral issue associated with small aquariums is enhanced aggression.

If you keep your fish inside a small aquarium, they can be aggressive towards others in the tank due to being scared and stressed.

The sad thing is that once your fish becomes aggressive, it becomes very difficult to transform them into being a friendly fish.

Therefore, the best thing to do is do everything in your power to prevent the fish from being aggressive.

Stunted Growth

Many fish keeping newbies commonly assume that fish only grow according to the tank’s size. This is far from the truth.

This is an entirely false misconception.

The myth emanates from people that keep goldfish. The fact that some fish are indeterminate growers simply means that their growth will continue until that time they will die.

If you are a fish keeper, you might have already known that when you place them inside a small aquarium, they will not grow.

If you want your fish to grow and live for long, you must keep them in a big sized tank. If you, make sure you follow the recommendations we have laid bare in this article.

The last thing you want to do is permanently damage the health of your fish due to the wrong choice of aquarium size.

What Size Is The Best For An Aquarium?

If you have read our article this far, you have already established that there is no best size for an aquarium. This issue is only solvable or answered on a case by case scenario.

Although that is the case, certain rules apply in most instances. Here are some of them:

  • Larger aquariums are easier to maintain. Maintaining involves things such as cleaning and water management.
  • Refrain from settling for an aquarium that is only ‘just large’ enough. It is prudent to choose a tank that is larger than you need so that your fish can thrive and flourish.
  • If you cannot get an appropriately sized fish, it is wise to settle for another fish species to put into your tank.
  • Always note that the bigger tank, the happier and healthier your fish will be.

Up to 10 Gallons

Professionals usually recommend five-gallon tanks as the least tank size if you want to keep one fish. All tanks under five gallons such as fish bowls tend to be extremely prone to accumulation of toxic chemicals, pH, and waste chemicals.

In addition to that, they lack a surface area that guarantees a healthy exchange of gas. The sad reality is that these tend to be stressfully tiny for even a single fish.

They do not have ample room to swim meaning your fish can turn out to be more susceptible and distressed to disease. However, while this is the case, it is important to note that small-sized tanks are usually not completely useless.

You can use them as vibrant homes for the dazzling aquatic algae and plant life. This can include the fashionable marimo moss balls or fluffy balls of algae.

It is worth noting that aquariums that hold ten gallons have their challenges too. Because of their tiny capacity, diligent pH monitoring and good filtration are necessary.

This goes miles to make sure that the environment does not turn out to be highly unstable for your fish.

11 to 20 Gallons

Tanks boasting the capacity of holding 10 – 20 gallons of water are not only pocket-friendly but also adequate for many fish owners. These make excellent choices for those looking for a manageable tank that does not take up much space.

On top of being decorative, you can use these aquariums as accessories to pimp your home décor instead of using them as centerpieces.

That being said, experts consider 20-gallon tanks to be small. You can only use them to house a few small fish.

Of great importance to note about these tanks is the fact that they need vigilant maintenance to make sure that there is proper chemical balance and filtration.

If you are a newbie that is interested in aquarium sizes with the ability to hold roughly half a dozen fish that are below 3” in terms of length, then you should consider a tank within this capacity range.

21 to 40 Gallons

Aquariums that boast a capacity of 21 to 40 gallons of water are excellent for housing roughly half a dozen to one dozen small fish for a happy and healthy life.

Their big volume makes sure that the quality of water is not overly temperamental. Thus, they tend to be more forgiving when it comes to less vigilant maintenance.

Although you will certainly pay more for such a tank, their big size is rewarding ultimately for sure. Once you set it up, and ensure you have installed its environmental and substrate features, there is no doubt that your aquatic world characterized by its inhabitants will always act as the focal point of your space.

More than 40 Gallons

If you want to display a huge variety of fish species, comfortably and beautifully, then you should consider a tank with the ability to hold more than 40 gallons of water.

What is for sure is the fact that big fish aquariums tend to be stunning when it comes to their appearance.

They make lovely and dazzling homes for fish of diverse sizes.

Although this is the case, a big-sized fish tank boasts its fair share of challenges. In addition to being extremely heavy, cleaning them can be quite challenging.

Because of their weight, you might have to buy a special stand. Installing structural reinforcements in the floor of your home can also be necessary to prevent damage.

The good thing is that most sellers of such big fish tanks sell them with an already installed in-built stand. What this means is that you have to keep in mind the weight and dimensions of the entire unit when you want to ascertain whether it can fit into your space.

Before buying a big aquarium, it is necessary to assess or evaluate all risk factors of the space in your home or business.

For instance, is the place a high traffic area? Can children access the place and severely damage your tank? These are some million-dollar questions worth asking yourself before settling on a big tank for your fish.

What is for sure is that these tanks make excellent choices for the savvy or experienced fish keepers that are familiar with the huge commitment associated with them.

Final Words

When it comes to the ideal size for your aquarium, it is worth noting that there is no perfect size for all aquariums. Plenty of factors need to be considered as we have established earlier on in this article.

For instance, the fish species plays a big in the size of the aquarium you should choose as some grow to be bigger while others do not grow to be as big.

It is my sincere hope that you found this article helpful. The next time you are looking for the appropriate fish count take size, you should consider all the information we have featured in this article.

If you feel that we have left out some important, please let us know in the comments section.

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