How Many Fish Can Live in a 30-Gallon Tank? A Guide for Beginners

Photo of author

by Jason Matthews



When you get into the fish hobby or even think about it, the first question that comes to mind is, “how many fish can I put in my tank?” The answer, however, is not as simple as a number. It depends on several factors, including the type of fish, their size, and your filtration system.

As a thumb rule, one inch of fish per gallon of water is a safe bet, and you could have anywhere from 1 to 30 fish, depending on the fish species and their care requirements. But that rule is not always accurate and requires variations depending on various factors.

This article will cover everything you need to know to ensure your little friends have a comfortable and safe home. Let’s dive into the Fish habitat.

Key Takeaways

  • The one-inch per gallon rule is a great general guideline when stocking your aquarium
  • When stocking your aquarium, be sure to take into account the aggression level of each fish.
  • The more fish, the greater the bioload, and water changes and filtration will be necessary to keep the water quality high.

One-Inch Per-Gallon Rule

Image Credit: huyphan2602, Unsplash

This is a popular thumb rule to make a decision “ How many fish can be kept under a specific gallon of water.”

This rule says that “One Gallon of water is required for one inch of fish”

However this rule has many flaws like;

Is it includes plants or not, with or without beneficial bacteria? Does one gallon of water include algae? grit or stones? And several others factors are questionable.

All these factors will play a role in whether one fish per gallon is possible or somewhat modification is required. So Let’s see what factors are these and how we can make a decision.

Factors Affecting the Number of Fish Per Gallon

A safe number of fish for an aquarium can be determined by considering a few different factors, and this is something every tropical fish keeper faces at one point or another.

1. Size of Fish

Image Credit: huyphan2602, Unsplash

This is a fact that, like all other living things, fish are also enthusiastic to grow. Just because a fish is small when you buy it doesn’t mean it will stay that size. 

In fact, some fish in the same family may grow to be much larger than other species. So, you need to take the estimated full growth of fish into account when deciding how many fish to put in your tank.

Another thing to remember is that not all fish grow at the same rate. Some will grow more quickly than others, so you need to be sure to monitor your tank closely and re-evaluate your stocking levels as your fish grow.

2. Swimming Space

Fish also need space to swim, and different fish have different space requirements. 

Even though the figures appear to be sufficient on the page but the tank may actually be too small for the fish to freely move about in. This is especially significant among active species and schooling ones.

For Example, Tiger botia are quite active and need more room to swim than other fish, while others, like bettas, are content to spend most of their time resting on the bottom or floating near the top of the tank.

Swimming space also varies according to swimming styles (Anguilliform, Sub-carangiform, Carangiform, Thunniform, and Ostraciiform) and the surface of swimming ( Top, Middle, and Bottom).

So, when deciding how many fish to put in your tank, consider the space each fish needs to swim comfortably.

3. Waste Load

Whatever the fish eat, they produce waste, which can build up in the water, causing ammonia and nitrite levels to rise. Beneficiary bacteria and plants use this waste to increase their populations and thus reduce the space for fish making the environment unfavorable.

The amount of waste a fish produces depends on the number and type of fish. Some fish, like Goldfish and Guppies, produce a lot of waste, while others, like bettas, produce very little. 

So, when stocking your tank, consider the waste products of each fish.

4. Filtration System

A good filtration system is essential for any aquarium and plays an important role in determining how many fish can live in your tank. The filtration system helps to remove waste products from the water, making it cleaner and safer for the fish, but you should be able to answer these queries;

Do this filter system working properly? Is it not more active to remove all the bacteria, including beneficiary bacteria? is it enough for 30-gallon water or not?

There may be a filter that is enough for one species but not for the other. So, it is important to check whether the filter you use is adequate for the number and type of fish you have in your tank.

5. Aquarium Plants

Image Credit: dpagayona, Unsplash

Plants are added to all aquariums because they help to keep the water clean and safe for the fish. They consume some of the fish’s waste products, helping keep ammonia and nitrite levels low. They also provide oxygen for the fish to breathe and offer hiding places for shy or timid fish.

So, you should consider the number of plants in your aquarium and their ability to consume the waste produced by the fish.

How many plants are required for one fish also varies from species to species? Guidelines for rough estimation can be seen in the video.

6. Surface Area

The surface area of the water directly impacts how many fish can be kept in an aquarium. The greater the surface area of water, the more gaseous exchange (especially oxygen), and the more fish can be supported.

A tank that is tall and thin may hold the same number of gallons as a tank that is short and wide, yet it will support less no of fish as less area for gaseous exchange.

According to the water surface area rule, every one inch of fish can be stocked under 12  square inches of surface area.

This one-inch rule is also fallacious, as it was designed to consider the slender-bodied fish, and this is not always the case. So, for wider fish, the rule will be one inch of fish under 20 square inches of surface area.

7. Water Temperature

The water temperature also plays a role in determining how many fish can live in your tank. 

The warmer the water, the less oxygen it can hold. This means that if you have a lot of fish in a tank, the water will become oxygen-deficient very quickly, resulting in suffocation.

So, when stocking your aquarium, consider the water temperature and the amount of oxygen the fish need.

8. Aggression level of fish

Some fish are peaceful and can be kept with other fish, while others are aggressive and will attack other fish.

Some fish, like bettas, are territorial and will only tolerate other fish if there is enough space for each fish to have its own territory while cichlids are more active and show aggression.

So, when stocking your aquarium, be sure to take into account the aggression level of each fish.

9. Aquarist Management

Image Credit: vikram, Unsplash 

According to some scientists, this is an aquarist on which the number of fish per gallon will depend.

Indeed the aquarist is the one who governs the number of fish in an aquarium. The more fish, the greater the bioload, and water changes and filtration will be necessary to keep the water quality high. So, If all the factors are accurate but inexperienced aquarists, that will result in fewer fish.

No. of Fish In 30 Gallon Water Tank

After considering the discussion, we can now put the fish in a water tank according to the one-inch one-gallon rule modification.

1. Cichlids

Cichlids are a large family of freshwater fish that come in various species and alluring colors. They have a classical fish-shaped body design and body size ranges from 2″ to 8″ but the largest cichlid, Boulengerochromis microlepis, can reach a size up to 3 ft (90 cm).

In a 30-gallon tank, you may keep from four medium-size cichlids like the peacock to six dwarf African cichlids, while a larger one cannot be kept in a 30-gallon tank.

2. Goldfish

These are freshwater fish with Slim-body, Golden brown color, and fan-shaped tails. When kept as a pet in the aquarium, they show retarded growth staying at 1-2 inches and never crossing the 6 inches. However, in the wild, they may reach 12 -14 inches.

A thumb rule for keeping Goldfish in an aquarium is 10 gallons of water per fish; thus, only three could be rare in a 30-gallon tank.

3. Guppy

Guppies are tiny tropical fish, also known as million fish or rainbow fish, with calm and laid-back behavior. They are available in a wide variety of colors and patterns.

The size of guppies varies from 0.6-2.4 so as a rule of thumb 10-12 Guppies can be kept in 30 dollar water tank.

4. Dwarf Gouramis

Dwarf gouramis are colorful, majestic-looking freshwater fish native to the slow-flowing rivers of south Asia. They are also known as labyrinth fish, which means they breathe straight from the air with a lung-like labyrinth organ.

They can reach the length of 2.5-4.5 inches, so a 30-gallon tank will hold  8-10 dwarf gouramis.

5. Tetra

Tetra is a common name for many small freshwater fish ranging from 1 to 2 inches.

These are active schooling fish and add splashes of colors to any aquarium. These hardly grow more than 1-2.5 inches so a 30-gallon tank can support approximately 15 tetras.

Some other fish sizes along with living No. in 30-gallon tanks are given in the table

Type of FishSize Range in InchesFish in 30-Gallon Tank
Molly0.5 -44-6
Angelfish5 – 121-2
Cory catfish1-49-11
Bristlenose plecos3-53-5
Cherry barbs1-210-12
Kuhli loach3-47-9


As you can see, we can easily decide the number of fish in a specific gallon of water. The trick is simple, apply one inch of fish for one-gallon water formula by considering all above mentions factors and calculate the figure. 

We can rule out that the number of fish will be approximately 3-4 times less than the per inch per gallon of water. 

Related: See our 30-gallon fish tank top picks

Frequently Asked Questions

How Many Tetras Should Be Together?

Tetras are schooling fish, meaning they make groups and move in the form of a group, so you should keep at least 6 neon tetras together in a tank so that they can make their community.

What Are Fish Breeds That Live Well Together?

Many fishes called “ community fish” will always make relationships and can live in a community with other species such as; Guppies, Gouramis, Tetras, Mollies, Loaches, Catfish, Corys, Danios, Platies, Plecos, Raspboras, Swordtails, etc.

What Fish Can You Not Put Together?

There is a number of fishes that are territorial and do not share their space with other species so these cannot be put together with other fish. Such as certain species of sharks, knife fish, Cichlids, mormyrids, and other territorial fish. 

Final Thoughts

The number of fish that can live in a 30-gallon tank depends on several factors, including the type and size of fish, surface area, the filtration system, the number of plants, and the water temperature. By taking all these factors into consideration, you can follow the one-inch per gallon rule to determine how many fish your tank can support.

Featured Image Credit: Corneschi, Unsplash


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

🐠 Get Your Free Ebook: A Beginner's Guide to Aquariums - Sign Up Now!

Learn everything you need to know to keep fish, including setting up your tank, choosing the right fish, and maintaining water quality. As a subscriber, you'll also stay up-to-date on the latest aquarium products, special offers, and discounts.

Your privacy is important to us. You can trust that we will never share or sell your information, and we'll only send you relevant content.

Leave a Comment