How Many Fish Can You Place in a Fish Tank?

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This article was co-authored by Craig Morton. Craig Morton is the CEO of Aquarium Doctor Inc. based in Huntington Beach California and servicing Orange County, Los Angeles County, and the Inland Empire. With over 30 years of aquarium experience, Craig specializes in creating custom aquarium designs along with aquarium installation, service, and maintenance.

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Like so many other things in life, when it comes to fish tanks, balance is key. Overstocking your tank can affect the health and happiness of your fish. Fortunately, it’s actually pretty easy to figure out how many fish your tank can hold. Just keep in mind that young fish will grow larger! To make it even simpler for you, we’ve answered a few of the most common questions that people have about choosing how many fish they can fit into their tanks.


How many fish can you put in a tank at once?

  1. Aim for about 1 gallon (3.8 L) for every 1 inch (2.5 cm) of fish. While there are many different types of fish, each with different needs, you can use this basic rule of thumb to make sure that your fish have plenty of room to move around in your tank. More room to swim means you’ll have happier and healthier fish. It’s also a good formula that will help keep your tank cleaner and prevent harmful substances like ammonia and nitrate from building up to dangerously high levels.[1] X Research source

    • So for example, if you have a tank that holds 20 gallons (76 L) of water, and you want to add some bottom feeder fish that measure about 4 inches (10 cm) long each, then you can add about 5 of them to your tank.

How many fish can I put in a 5-gallon (18-liter) tank?

  1. About 4-6 fish depending on how big they are. Your best bet is to avoid having more than 5 inches (13 cm) of total fish length. To figure out your total fish length, simply add up the lengths of the fish you’re thinking about adding to your tank. But, keep in mind that if you purchase young fish, they may grow to larger lengths and may outgrow your tank.[2] X Research source

    • In general, fish that can grow larger than 2 inches (5.1 cm) aren’t recommended for 5-gallon (18-liter) tanks. They just won’t have enough room to swim around and be happy.
    • 5-gallon (18-liter) tanks are best suited for fish like micro rasboras, guppies, and betta fish.
    • Avoid putting more than 1 betta fish, though. They’re super territorial and may start up a mini-fight club if you put multiple in the same tank.

How many fish can you put in a 10-gallon (38-liter) tank?

  1. You can fit about 5-10 small fish, depending on their size. Your best bet is to start off small if you’re just getting your tank set up. As you get better at keeping fish and your tank has matured into a thriving environment that you’ve managed to keep clean and healthy, you can add up to 20 small fish as long as your tank can handle it.[3] X Research source

    • A few fish that can thrive in a 10-gallon (38-liter) tank include tetras, corydoras, sparkling gourami, and guppies.
    • Keep in mind that some small fish can grow into larger ones. For instance, a tiny goldfish can grow up to 6 inches (15 cm) in a year.

How many fish can I put in a 55-gallon (200-liter) tank?

  1. Just make sure you don’t exceed 55 inches (140 cm) of fish. It’s nice to have a larger tank to work with! You can create a really interesting underwater environment, but the key is to avoid overstocking it and stressing out your fish. Stick to the classic “1 gallon (3.8 L) for 1 inch (2.5 cm) rule” and don’t overdo it.[4] X Research source

    • For instance, you could fit 10 small guppies, 5 bottom feeders that measure about 4 inches (10 cm), 5 rainbow fish about 2 inches (5.1 cm) each, and 3 larger parrot fish that measure 5 inches (13 cm).

What happens if I have too many fish in my fish tank?

  1. Your tank will be harder to keep clean and it could harm your fish. Having too many fish means your tank will have a lot of fish waste, which can make your tank dirty and harder to keep clean. Additionally, having too many fish can produce too much ammonia, which can actually cause your fish to get sick and possibly die if the levels get too high.[5] X Research source

    • Even if you have a really strong filter that’s designed to remove ammonia, your tank can still have higher levels of nitrate, which can affect the immune systems of your fish and make it easier for them to catch and spread diseases.
    • Your fish can also get really stressed out if there isn’t enough room for them to swim around, which can affect their immune systems and overall health.

Is there a fish calculator you can use for your tank?

  1. Yes, visit If you’re looking for a simple and more accurate way to calculate how many fish you can fit in your tank, use this online calculator. Enter information such as the measurements of the fish, your tank’s specifications, and the type of water your tank holds to get an estimate.[6] X Research source

    • Because some fish have different oxygen needs, you can specify whether you have slender fish (such as tetras or danios) or full-bodied fish (such as goldfish or oscars) to predict how many you can safely add to your tank.

What’s the lucky number for fish?

  1. The number 9 is used for prosperity in feng shui. It’s considered an auspicious number and symbolizes your wealth and a happy, long life. Many people who follow or practice feng shui will have 9 fish in their fish tank or pond in order to channel this positive energy.[7] X Research source

    • According to feng shui, the best combination to have is 8 goldfish and 1 Arowana fish, which symbolizes good luck.

Community Q&A

  • QuestionCan axolotls live with other animals?Aussie JCommunity AnswerMost choose to keep them separately as they require different conditions than most other aquatic animals.
  • QuestionI have a foxface and the other night while watching him I saw a bug/parasite looking thing enter through his gills. The fish is still alive now. I was wondering how to get rid of that parasite?Titans – Gaming and moreCommunity AnswerTry quick ich X, it is a medicinal solution with a small amount of salt that is harmless to the fish, and should kill the parasite.
  • QuestionHow many fish like harlequins can fit in a ten liter tank?cymbospondylus 846Community AnswerOne harley needs 5 liters so if you want to keep them you can only get 2 harleys although they are 5 cm.


  • A quality filter and tank supplies such as an air stone can help keep your tank cleaner, but it won’t actually increase the number of fish you can keep in your tank.⧼thumbs_response⧽


  • Avoid choosing sick or unhealthy fish to add to your tank. They could introduce diseases to your other fish.⧼thumbs_response⧽


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About This Article

To know how many fish you can place in a fish tank, follow the general rule that you’ll need a gallon of water for every inch long your fish is. For example, a 3-inch fish would need 3 gallons of water just for itself. You should also take into account the types of fish you’ll be keeping, since larger, aggressive fish will need more room than smaller ones. Moreover, you can put more fish in your tank if you choose species that like to swim at different levels, such as beta fish, which swim at the top. For tips on how to calculate how large your tank is, keep reading!

Reader Success Stories

  • “As a beginner, it helped me understand how to measure how many fish I can safely keep.”

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