How Many Fish in a 15 Gallon Tank? (15 Gallon Stocking Ideas)

how many fish in a 15 gallon tank

If you have ever wondered how many fish in a 15 gallon tank is best, you will be surprised the answer is fairly straightforward. The rule is a single one-inch fish for every gallon of aquarium real estate. Hence, a 15 gallon aquarium should accommodate 15 one-inch fish or one 15-inch fish.

Of course, the number of fish per gallon is relative. After all, most aquarists love combining different fish species in the same tank. Continue reading to learn more.

How Many Fish Can I Put in a 15-gallon Aquarium?


The ideal number of fish for a 15 gallon tank depends on the fish species, including their maximum adult size. Aquarists observe the “1:1 rule,” reserving an inch of fish body length for every gallon of aquarium water.

Since we have a 15-gallon tank, we can put 15 inches of fish length (wholly or combined).

As simplistic as this rule is, it hardly is in real life. For example, many 15 gallon fish tank ideas combine different fish species in the same tank. Does that mean we can combine nine one-inch Tetras with two three-inch Tiger barbs in the same tank?

This combination will have the Tiger barbs lunching on the Tetras because of the former’s aggressive nature.

Although the 1-inch:1-gallon rule applies, we must be cautious when developing 15 gallon stocking ideas.

Overstocking can be an issue, reducing oxygen for all fish while increasing fish waste that might overwhelm the filtration system.

There’s also the issue of live rocks and corals that can take up precious fish tank space. Corals guarantee aquatic balance by forming symbiotic relationships with other fish tank inhabitants.

Meanwhile, live rocks are necessary for biological filtration, leveraging invertebrates, algae, and bacteria for efficient waste processing and ensuring an effective nitrogen cycle.

Live rocks and corals limit the space for fish to roam in the tank. Hence, the 1:1 rule might be inapplicable, especially with a small 15-gallon tank.

So, what fish should you put in a 15-gallon tank?

What Fish Should I Put in a 15-gallon Tank?

Aquarium real estate is a concern with 15-gallon fish tanks. Hence, you might want to consider small tropical fish species.

The following are ideal freshwater fish for 15 gallon tank.

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  • Neon Tetras


Adult Neon Tetras can grow up to 1.5 inches, allowing you to put about ten fish in the 15-gallon tank. These fish are colorful, making your aquarium look more vibrant.

  • Guppy fish


Almost everyone loves guppies. These brightly-colored fish with Betta-like tail fins can grow up to 2.4 inches for females and 1.6 inches for males. Hence, you can stock six females or nine males in a 15-gallon tank or use a combination of males and females.

  • Zebra danios


With their purple stripes running along their bodies, Zebra danios make perfect fish for 15-gallon tanks. They can grow up to 2.5 inches, although some might reach three inches. Hence, you can put five to six Zebra danios in your 15-gallon tank.

  • Dwarf puffer fish


We recommend this fish for beginner aquarists who want a unique fish to showcase in the aquarium. It can grow up to 2.5 inches and loves to feast on snails, shrimp, and bloodworms.


However, these fish are aggressive towards other species. So it is best to keep six Dwarf puffer fish in the 15-gallon tank by themselves.

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  • Dwarf gouramis


This fish is perfect for a 15-gallon tank if you do not mind its 3-inch maximum adult size, meaning you can only put four to five in the aquarium. The best part is this fish has a calm demeanor, featuring a brilliant blue hue.

  • Mollies


This live-bearing fish can reach up to five inches long in adulthood, limiting the number of mollies you can put in your 15-gallon tank to about three to four. However, you can pick small varieties, such as the Sailfin molly (2.5 to 3 inches) and the Common molly (2 to 4 inches).


Here are our fish suggestions for a 15-gallon saltwater aquarium if you have this setup.

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  • Gobies


These saltwater fish range in size from one inch to six inches. You can pick the Eviota (1 inch), Green Clown (1.75 inches), Neon, Yellowline, Sharknose, Hi Fin Red Banded, Red Head, and Citron Clown (all 2 inches). You can have as many as ten Gobbies in your 15-gallon tank.

  • Clownfish


Although larger than Gobies, Clownfish make excellent inhabitants for a 15-gallon saltwater aquarium. You can put four to five Ocellaris and Percula Clownfish (3 inches) in the tank or three to four Pink Skunk, Saddleback, and Orange Skunk Clownfish (4 inches) in the aquarium.

  • Cardinalfish


Here is another small saltwater fish to consider for your 15-gallon tank. Cardinalfish has 200 species ranging in size from two to eight inches. We recommend picking the small varieties to enjoy six to seven Cardinalfish in your aquarium.

  • Damselfish


Easy to care for, Damselfish is a favorite among beginner saltwater aquarists. These colorful fish range in size from 2 to 12


We recommend the two-inch varieties (Springer’s or Blue Sapphire Damselfish) or the 2.25-inch species (Talbot’s Damselfish), so you can put six to seven in your 15-gallon tank.

You can also pick Azure, Blue Devil, Yellow tail, Lemon, Pink Smith (all at 2.75 inches), and Neon Damselfish (2.5 inches).

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  • Blennies


The bottom-dwelling Blenny fish is another worthy fish to put in a 15-gallon saltwater tank. Although most will grow up to less than four inches, some can reach five. We do not recommend putting more than three because these fish also need rock to promote healthy growth.

  • Basslets


Growing a little over three inches, Basslets make excellent saltwater fish for 15-gallon tanks. They have a vibrant purplish body that should give your aquarium the character it needs.


Most saltwater fish are big compared to their freshwater counterparts. Moreover, adding corals and live rock to the aquarium can lessen the space for these aquatic creatures. Hence, we do not normally recommend putting saltwater fish in a 15-gallon tank unless you are okay with only a few.

To be safe, you should deduct 10 to 15% of the tank volume to account for decoration. So a 15-gallon tank would be viewed more like a 13.5 or 12.75-gallon one.

Which Fish Can I Put Together


Putting the same species in a 15-gallon tank is the obvious choice for beginner aquarists. However, you can create fish combinations to turn your small aquarium into a thriving community.

Please check the following fish combos for a 15-gallon tank.

1. Combo 1

  • 1 Dwarf gourami
  • 2 to 3 Guppy fish
  • 4 to 5 Neon Tetras

2. Combo 2

  • 1 Siamese fighting fish or Betta fish
  • 4 Panda Corydoras catfish
  • 8 to 10 Harlequin rasboras

3. Combo 3

  • 1 male and 1 female Electric Blue ram fish
  • 4 to 6 Albino Corydoras catfish
  • 8 to 10 Ember Tetras

4. Combo 4

  • 1 male and 1 female Dwarf gourami
  • 8 to 10 Cherry shrimp
  • 8 to 10 Glowlight rasbora

5. Combo 5

  • 1 Dwarf gourami
  • 6 Platy
  • 5 Danio
  • 1 male Betta
  • 3 Rasbora

You can try the following combinations for your 15-gallon saltwater tank.

  • 1 Basslet
  • 1 Goby
  • 1 Blenny
  • 1 to 2 Cardinalfish
  • 1 male and 1 female Clownfish

Frequently Asked Questions


What are the dimensions of a standard 15-gallon fish tank?

The dimensions of a 15 gallon tank vary across two standard types or designs: long and tall. High or “tall” 15-gallon fish tanks are 20 inches long, 10 inches wide, and 18 inches high.

On the other hand, a 15 gallon long aquarium is longer by four inches (24 inches), wider by two inches (12 inches), and shorter by six inches (12 inches) than a 15-gallon high tank.

You can also use an online calculator to determine the ideal dimensions of a custom-built 15-gallon fish tank specific to your needs.

Is a 15-gallon tank ideal for saltwater fish?

No, a 15 gallon saltwater tank is not ideal for keeping fish. Experts say the idyllic tank size for saltwater fish is 55 to 75 gallons, although the smallest recommended saltwater aquarium for beginners is 20 gallons.

It is worth noting that saltwater contains 20% less dissolved oxygen than freshwater, requiring a larger-sized tank to maintain stable oxygenation for the aquatic inhabitants. Remember that saltwater aquariums look best with live plants and corals taking up space.

However, you can still put saltwater fish in a 15-gallon tank, provided they are small. One to six saltwater fish should be fine.


Determining how many fish in a 15 gallon tank to have can be tricky, especially for beginner aquarists. Although the one-inch-fish-to-one-gallon-water rule can help, the final fish count depends on the aquarist’s understanding of fish characteristics and behavior.

Erring on the safe side, we recommend going for a lower number than the rule. For instance, a 15-gallon tank can accommodate 15 one-inch fish or one 15-inch species. However, you can stock the aquarium with 12 one-inch fish or one 13-inch fish.