How Many Fish Should You Put in a Fish Tank?

Updated on August 7, 2019
landocheese profile image

I enjoy having pet fish and maintaining my aquariums. I'm happy to share my advice on how to make a tank safe, healthy, and beautiful.

To keep this pet healthy, give him enough space.
To keep this pet healthy, give him enough space. | Source

Work on the Tank Before You Buy the Fish

The first thing many new aquarium owners do is run out and buy a bunch of colorful fish, but if you don't know how many fish you should put in your new fish tank, or how many is too many, you may end up flushing some down the drain within a short time.

It is far better to do your homework first before stocking a fish tank with new fish. Your aquarium is an underwater ecosystem, and the chemical makeup of the water is imperative to sustain your fish and keep them healthy. Let's take a look at how many fish your fish tank can safely hold.

Hold on!

Before you head to the store, read on! There's more to learn to save your fish!

Number of Fish in a Tank

The number of fish that will fit in your aquarium depends on the aquarium size. That may seem obvious, but there's just a bit more too it than that. While there is no perfect way to determine the actual number of fish that you should stock in your tank, there is a general rule, and here it is:

How Many Fish per Tank?

1 inch (of fish) for every 1 gallon of water

That sounds very simple, right? Wrong! The problem is that this overly simplified statement is nearly impossible to apply for any new fish tank owner. This is because fish that we stock a fish tank with are usually babies, and they will grow. I have never seen a pet owner at the fish store carefully calculating the mature size of the fish being purchased. This means trouble down the road for some of them.

For a general estimate, in a typical 10-gallon aquarium, you can easily support 6–8 fish, as long as none are whoppers. A 20-gallon fish tank doubles that. Try to mix it up with fish that like to be low and others that like to swim high, and your aquarium will feel plenty full.

However, the only way to create a truly healthy underwater environment and to determine how many fish your tank will hold is to grow into it over time. Here is the process outlined for how to do just that.

This guy looks innocent now, but how big will he get?
This guy looks innocent now, but how big will he get? | Source

What to Do Before You Add Fish

One of the most important steps in setting up a new fish tank is to prepare the water, and water fresh from your sink is not a great place for new fish. First you need to go through a process called aquarium cycling to setup this new ecosystem.

What Is Aquarium Cycling, and How Do I Do It?

To cycle your aquarium is to get the bacteria to start working in a way that stabilizes the water and prepares if for fish.

  1. For starters, after you setup your tank, let it run for a few days with no fish to get water flowing through the filter. Bacteria will begin to establish themselves, including good ones.
  2. This is also the time to adjust the temperature to get it just right, so you don't have to experiment with your new heater at the expense of your fish. A range of 75-80 degrees is a good target, so shoot for the middle of that range and you will be pleasing most tropical fish.
  3. Finally, get some fish food, a small skimmer, a water test kit, and a siphon ready so that you can be a responsible fish owner once the little guys arrive. I use a siphon that starts the flow of water easily with the squeeze of an air pocket built-in to the hose, and I love the water test strips for ease of use.

Danios make good first fish.
Danios make good first fish. | Source

Adding the First Fish to a Fish Tank

Now that you're ready, go ahead and head to the pet store. The job of the first few fish is to further establish the environment. It can be tough in there, so send in some tough fish. In my opinion, your first fish should be a Danio. They are attractive but tough. You can also probably find some at a very fair price.

Avoid the frilly, pretty fish right now. They are for later, when the tank is well established. You need a fish that is go to go at this point, and Danios are a perfect choice.

  1. Start with 2-3 fish, assuming you have at least a 10 gallon tank. If your fish tank is smaller than that, a single fish can get things started.
  2. When you get this fish home, feed lightly. Too much food is a really bad idea. You may notice that it takes the fish a day to eat well.
  3. Feed no more than the fish will eat in a minute or two, or you'll be making the environment worse.

Fish Water Testing Kit

Now that you're on your way, start testing the water with an aquarium water testing kit of some kind. You can find all sorts, but I personally use the testing strips, which seem to do a nice job, and I love the convenience of using them. With an all-in-one test kit, you can monitor the water for these key components:

  • pH Level: This is a measure of acidity or alkalinity in your water. Your fish can take the outer range of either, but they don't like rapid changes in it. The desired range is 6.5-8.2.
  • Alkalinity: This measures how likely it is that you can maintain a level pH level in the tank. The optimal range is 120-180 ppm for a freshwater tank.
  • Hardness: This measures things like calcium in your water. Too hard or too soft can be harmful to fish. The best range is between 50-150 ppm for nearly all tropical fish.
  • Nitrate and Nitrite: These are basically made from the waste of bacteria that help to keep ammonia under control. It's best to have a level under 40 ppm for nitrate and .5 ppm for nitrite.
  • Ammonia: This is basically created from the waste of your fish. In a new aquarium it may register at a bad level, but once the good bacteria get going this number should dwindle to nothing. The best level for ammonia is zero.

Start right away and measure your water for these things weekly and write it down. Then, you can track the progress and decide when it's safe to add a few more fish. Basically, once your chemical levels get under control, you can add more fish, 2-3 at a time, until you think your fish have hit the 1 inch per gallon of water cap - after you have projected their final size, of course.

Now you can add some neons, or a bottom feeder.
Now you can add some neons, or a bottom feeder. | Source

How to Keep a Fish Tank Clean

Once you have done the work to cycle the aquarium, tested your water routinely, and have slowly added new fish, make sure you keep the tank running well. Here are the recurring tasks that should be on your calendar to keep the aquarium in tip-top shape.

  • Daily Tasks: Feed your fish twice per day, but only as much as they will eat in 1-2 minutes. Make sure the temperature is still right, especially if seasons are changing or the location is near a window.
  • Weekly Tasks: Run your water tests and write the results down. Use your siphon to remove 10% of the water from the tank and fill it will fresh water treated to remove the chlorine.
  • Monthly Tasks: Check your filter and replace it if needed, and it probably is. Use your siphon to vacuum the gravel to pick up loose junk. Replace lost water with fresh water treated to remove the chlorine.

Be Patient and Keep Testing the Water

Hopefully you have learned that, as important as how many fish you should put in your fish tank is, the method with which you add them is even more important. The 1 inch to 1 gallon rule is an old rule of thumb, but it's not perfect. The best way to measure is to keep testing the water and to add fish slowly. Believe me, when you've hit a limit, your water test will show it quickly, but it's best to avoid that moment.

Remember, cycle your tank, add just 2–3 fish to start, and test the water. Most of all, be patient. You may want a fish tank full of fish, but in the long run, slow and steady will win the race. Make every fish count, because by caring for your water carefully, they should be in your tank for a long time. Good luck!

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


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    • profile image


      2 weeks ago

      If i were to get 7 white cloud mountain minnows, 6 Cory catfish, 5 Zebra danio, and 4 Neon tetra how big of a tank do you think i would need?

    • profile image

      David Rothwell 

      8 months ago

      Do cicchlids and gauromy fish get along?

    • profile image

      Julie Reagan 

      16 months ago

      I am getting 20 fish, what tank size would work?

    • profile image

      Joey Burgess 

      6 years ago

      I am one of the new hobbyists and just doing my research. I am very interested and very excited. I have been in a bad car accident and it is the way my doctor wants me to keep my stress down. I want an aquarium so bad, I love them. I read this article and it was educational and it had some good pictures. Not to mention, it was great advice. GOOD LUCK EVERYONE!

    • Rob Winters profile image

      Rob Winters 

      7 years ago

      Great practical and sensible advice for prospective new fish owners.Up & Useful. Great images too :-)

    • Peggy W profile image

      Peggy Woods 

      7 years ago from Houston, Texas

      Aquariums are so beautiful to have and own. They do however need good maintenance. Enjoyed reading this hub and it offers good advice for those just starting out. Up, useful and interesting votes. Will pin this hub to my PInterest board.


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