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How Many Fishes Can you Keep in your Fish Tank?



How Many Fishes Can you Keep in your Fish Tank

How many fish can you keep in a Fish Tank? – This is a great question to ask before you add any new fishes into your fish tank because a lot of people think that they can just add any number of fishes into their fish tank and that is totally wrong.

This is because when a fish tank owner gets a new fish, they are usually small babies which in time will grow in size.

A fish tank has it’s own unique marine ecosystem and in order to make sure that your fishes are living in a healthy ecosystem, you have to take care of a lot of things. One of them is fish population.

Knowing how many fishes you can keep in your fish tank can be tricky. There is a general rule of thumb that is – “One 1 Inch Fish for every 1 Gallon of Water.”

That is 1 Gallon is approximately 3.8 Liters of water so for every 3.8 Liters of water, you can keep One 1 Inch fish of 2.5 cm.

This is a rule that is generally followed by everyone but the number is not exact and there are several more reasons to consider.

Today, in this article, we’ll go over How many fishes should you keep in your fish tank? and we’ll talk about additional important things that you should know while determining how many fishes you want to keep.

Factors Affecting the Number of Fish in a Tank

  1. Tank Size: When selecting the size of your tank, it’s essential to take into account the amount of fish that you can safely keep in it. A larger tank will offer a healthier habitat for your aquatic friends and allow more room for them to swim and grow, whereas a smaller one could become overcrowded if not monitored closely.
  2. Water Quality: Maintaining a healthy level of water quality in the tank is critical to ensure that fish remain healthy and are able to thrive. Poor water conditions can lead to stress, illness and even death amongst your fish population; significantly reducing their lifespan when not properly addressed.
  3. Fish Species and Size: When selecting which fish to stock in an aquarium, it is essential to consider the size and needs of each individual species. After all, some types may grow larger than others. Furthermore, the tank’s capacity should be taken into account when deciding how many fish should live there – both for their comfort levels as well as your own.
  4. Fish Behavior and Compatibility: When determining the number of fish in your tank, it is essential to not only consider the size and filtration system but also their behavior. Some species may be aggressive or territorial while others are sociable; thus, selecting compatible fishes creates a balanced environment that promotes harmony and wellness.

How does Fish Population and Other Variables Affect your Fish Tank Ecosystem?

While for a total newbie, keeping as many fishes as he wants in his fish tank seems totally logical, but that can create lots of problems for the fishes in the future.

You might be wondering that, by keeping too many fishes in a fish tank, you will face the problem of space in the future and yes, that is true.

But the problem doesn’t end there. This is because every fish tank has it’s own unique marine ecosystem and you as the owner is responsible for keeping the ecosystem in a healthy state.

That’s where keeping optimal levels of fish population is extremely crucial for your fishes.

Here are four things that can negatively impact your Fish Tank Ecosystem.

1. Fish Waste Load

The first one of-course is waste load. The biggest reason why keeping too many fishes in your fish tank can be unhealthy for your fishes is due to waste load.

When fish eat food, they create waste that is broken down by the plants in your Fish tank. While, there are also bacteria’s that help in the process of waste management.

When the waste load in your fish tank is too much, it negatively impacts the water quality in your fish tank. Due to this, your fish may get ill or even worse die.

As every fish tank has a Nitrogen Cycle which is all about how the nature creates food for the fishes, which the fish eat and then they create waste and how the waste is managed and broken down by plants.

By keeping too many fishes in your fish tank, the Nitrogen Cycle in your fish tank starts to get too toxic which is not good for your fishes. I will touch more on the topic of Nitrogen Cycle in my future articles.

2. Swimming space and Fish Size


Size and space matters and the amount of swimming room is very important factor to consider. The size of your fish tank and the kind of fish you wish to keep in your fish tank matters a lot.

A Gold Fish for example can grow upto 8 inches long that is 20 cm in length, while the other types of fishes can be between 5 cm to 30 cm in length. In this case a 20 to 25 gallon fish tank would be appropriate to accommodate your fishes.


One should do their due diligence and properly research the minimum tank size required for the fish you wish to keep.

There are so many species of fishes like the Zebra Danios which are extremely active fishes and they require a lot of swimming space even though they may only be 5 cm in length.

By the time, your fish matures, they will grow upto 2 to 3 times their original size and you should have enough space in your fish tank to keep your fishes.

3. Nirtrate Levels, Plants and Filtration

Every fish tank has it’s own Nitrate levels depending upon several factors. The first thing you want to consider is at what Nitrate level do you want to change your water.

The reason why we change the water of the fish tank every week is to control the nitrate levels. The water changing cycle will take care of the ammonia and nitrates in the water.

Now the number of fish you wish to keep in your fish tank depends upon how you handle the nitrate levels in your fish tank.

The more fish you add to your fish tank, the nitrate levels will increase accordingly. As we know, fish creates waste loads that increases the toxicity levels of a fish tank. Which means, you will need to frequently check the nitrate levels and may have to change the water in a few days or week, if the nitrate levels rise.

Along with the Nitrate levels, you need to see how many live plants do you keep in your fish tank and what type of filtration do you use.

Therefore, in order to determine the amount of fish you wish to keep in your fish tank, you will need to consider the above variables.

If you wish to keep 10 fishes, you have to keep in mind, how much nitrate levels will your fish tank be at after a week and then manage the water changing cycle.

4. Aggression Levels

Fishes are indeed living organisms and just like humans, they share the same kind of values and behavior that we as humans posses. An Aquarium has it’s own marine ecosystem and there are different species of fishes.

Some fishes may be very aggressive in nature while some may be more passive. You have to keep in mind, what variety of fish you are keeping in your fish tank.

This is because Fishes that are dominant may show aggressive behavior towards other species of fishes due to which they may try to control a certain part or territory in the fish tank.

In order to create a healthy marine ecosystem, you can add lots of decorations and plants in your fish tank so that, the passive fishes have enough line of sight to hide from the aggressive ones.

How to Determine How many fishes to Keep in your fish tank?

The general rule of thumb as we know is One 1 inch fish per 1 Gallon of water, however, this is not an accurate method to determine the amount of fish that you can keep in your fish tank. That is because, number one, we all have different sizes of fish tanks and every fish tank will have different levels of Nitrates.

Now our job as a Fish tank owner is to control the nitrate levels because that is one of the most important thing to do.

The easy method is to first measure the Nitrate levels in your fish tank. The nitrate levels should be below 40 ppm and if it’s below 20 ppm even better.

What you can do is use the Nitrate levels to determine how many fishes to keep in a fish tank. For that, firstly I assume that your water changing cycle is once per week. If that is good for you then, the next thing you need to note down is fish tank size and Nitrate Levels.

Every week, when you change the water of your fish tank say you remove 50% of the water, you reduce the nitrate levels by 50%.

Now say, you want to keep 20 Fishes in your fish tank that has accommodates of 20 Gallons of water.

You can start by first choosing which species of fishes you want to keep and finding out their aggression level, compatibility, diet and so on.

After that you can add 5 – 7 fishes every 2 weeks, and in doing so, you should measure the nitrate levels.

This way, you will be able to determine, how much waste load your fish produces in a week and what is the increase in the nitrate levels.

Next, you will have to measure the nitrate levels for 2 – 3 weeks and be certain that your fish tanks, nitrate levels are below 40 ppm.

After that add the next batch of 5 – 7 fishes and repeat the nitrate levels measurement process and water changing cycle.

If you find that by adding more fishes, your nitrate levels are exploding to higher levels, then you may have to reduce the number of fishes, add more plants and bacteria or made adjustments to your water changing cycles.

So, this is a very efficient way to determine how many fishes you can keep in your fish tank. However, you have to keep in mind that your fish tank is an entire marine ecosystem where a lot of things happen. Fishes can reproduce creating more off springs that can increase the waste load. While, overtime, your fish tank can change a lot and sometimes, it can be difficult when the population increases. Especially, when the plants grow which helps in reducing the waste load but reduces the swimming space.

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