How Many Fish in a 10 Gallon Tank? – 9 Essential Factors

As a beginner aquarist, you feel immense joy to provide a suitable environment for your fish. But suitable environments need detailed care.

Fishes like any other pets have specific requirements. One of the first questions that come to mind here is “How many fish in a 10 gallon tank?”

10 gallon tanks are by far the most accessible and popular tank sizes out there. Making them is an ideal choice for beginners to pros in fishkeeping.

Many even get 10 gallon tanks as gifts. That is when the type or number of fish suitable for habitation in such a tank becomes the focus.

The additional factors cause many variations that affect your fishes’ health. Leading to so many unanswered questions, we are here to answer them all.

The famous “One Inch Per Gallon Rule”

The one inch per gallon rule is one of the most widely propagated rules in fishkeeping. Also called “the rule of the thumb”, it suggests that your tank can hold one inch of fish per gallon nowhere mentioning the kind of fish that you are keeping.

Is the “One Inch Per Gallon Rule” Feasible? If not, Why?

At first glance, it seems like a reliable rule but in reality, it almost never works and is often harmful.

Different fish vary widely from their dietary consumption to waste generated to space for active habitation.

Due to this, some fish for a 10 gallon tank like Neon Tetras might find it suitable but other’s bio mechanism might overwork filters installed in such a tank.

So, if this rule is indeed a myth, how many fish can you actually keep in a 10 gallon tank? Read on to find out.

The Real Answer to “How many fish can I keep in a 10 gallon tank?”

The number of fish that can live comfortably in a 10 gallon tank is directly tied to the species of fish that you are keeping.

As 10 gallon tanks are relatively small, small-sized, non-aggressive fish are best suited for living in these tanks.

Some suitable fish for a 10 gallon tank are: Least Killifish, Guppies, Dwarf Gourami, and Betta Fish.

When selecting such fish, always keep in mind their adult fish size. If you are a beginner aquarist, one small fish per gallon of water is the correct measure for you.

Later on, when you are a skilled aquarist and are able to provide your fish with specialist care, you may aim for two small fish per gallon.

Talking about tank sizes reminds us to explore other available options as well. What about the number of fish for a 20 gallon tank?

So, how many fish should you have in a 20 gallon tank?

A 20 gallon tank provides you with exciting possibilities that a 10 gallon tank fails to deliver.

Firstly, it gives you the opportunity to create a healthy ecosystem by including living plants in the tank. Secondly, it allows you to stock various good community fish.

Some suitable fish for a 20 gallon tank are: Cory Catfish, Swordtail, and Platy

Again, whether it is a 10 gallon tank or a 20 gallon tank, the number of fish is dependent on its species.

Moreover, it is also dependent on a variety of factors that you may have failed to take into account.

Major Factors to keep in Mind for Accommodating Fish

A subset of the diverse factors affecting the growth and health of your fish are listed below.

Before accommodating fish, you must take care of each of these factors to be successful as an aquarist.

1.    Dimensions of the Tank

When housing large fish or fish with high relative activity, tank dimensions need to be taken special care of. As these are fish with unique requirements, a spacious tank is ideal for them.

The tank should leave enough space for the fish to swim about after it’s fully grown.

55 gallon tanks are often termed as the minimum requirement for large fish that like to school.

Being as wide as 12 inches, fish of the Cichlidae family school well in these tanks.

2.    Tank Shape

Tank Shape is rarely ever talked about. The most common fish tank shape is rectangular.

Ever wondered why not circular or curved? Curved fish tanks distort the view of the fish being kept in them which can lead to behavioral issues.

But rectangular fish tanks not only eliminate this problem but also provide maximum surface area for higher oxygenation.

3.    Actual Volume of Tank vs Perceived Volume

The main difference is that the perceived volume of your aquarium is much smaller than its actual volume. Plants, rocks and other decorations displace water and bring down a tank’s effective volume.

Fishes too take up space which water would fill.

Overall, the actual volume gets reduced by a factor of about 15% including all physical and chemical processes in an aquarium.

4.    Surface Area of Water

Oxygen exchange occurs at the surface of the water so a higher surface area results in higher oxygenation rates.

Higher oxygenation helps improve waste decomposition as well.

Studies have shown two fish tanks with the same surface area would be ideal for holding the same type and number of fish regardless of their height.

5.    Fish Shape and Size

The fully grown fish size must lie within the spatial dimensions of your tank.

The fish shape helps you determine the width or the height of the tank required.

6.    Fish Behavior: Is your fish species aggressive?

Many schooling fish like Neon Tetras feel threatened when kept isolated or in a pair.

Many fish like Bettas act aggressively towards other Bettas. Certain fish act predatory towards smaller sized fish.

On the topic of how many fish per gallon, if you overcrowd it, say keep 5 small fishes per gallon, a stressed environment is bound to develop between the inhabitants.

There is no magic formula to sort this problem, only dedicated research on the fish that you are keeping helps idealize a calm, habitable environment.

7.    Fish Activity: How much space does your fish require?

Naturally hyperactive fish require a spacious environment to swim about. But when your fish shows erratic swimming patterns or paces about, it indicates stress.

This stress may be caused by fish incompatibility or chemical imbalances in the aquarium.

It is advisable to check out the pH balance and algae build-up in the tank.

8.    Fish Waste

To maintain a healthy fish waste cycle, you must initially feed your fish sparingly. Customize this process to fit their biological need but never overfeed.

Overfeeding upsets the ecosystem by polluting the water. Also, it causes a harmful rise in nitrite and ammonia levels.

As much as cleaning is crucial, over-cleaning makes the biosystem in the aquarium unstable, so avoid it.

9.    Filtration

An ideal filter should provide a turnover of 4-6 times your tank volume. So for a 10 gallon tank, 40-60 gallons per hour filtration is perfect.

Except for this, a 25% water change every week or every two weeks is healthy. A nitrite test kit might also come in handy to keep nitrite levels in check.


Be it fish for a 10 gallon tank or fish for a 20 gallon tank, the number of fish depends on various factors linking to their adult size, community behavior and bio load.

Do a detailed research on your species of fish before stocking them.

Avoid following one golden rule and follow those that suit your aquarium to provide a healthy and happy environment for your fish.   

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