How Many Fish Can You Put in Your Tank?

Overstocking is a common problem with fish tanks. Here we tackle the question of just how many fish can I put in my tank.

Jun 7, 2024By Kenny Jarvis
how many fish can you put in your tank

You get a new tank, and you're excited about setting it up. You've got everything you need, start to get it cycled and your next step is choosing what fish to put in there. Here's where an extremely common mistake can happen with new fish owners.

Many will accidentally overcrowd their tank in the excitement of wanting as many fish in there as possible. However, this can lead to poor water quality and stressed fish. Let’s dive in to just how many fish you should put in your tank.

Understanding the Aquarium Ecosystem

overstocked cichlid tank
Source: YouTube

Before looking at how many fish you should put in a tank, it’s important to understand the environment they are living in. While these fish may originate from vast bodies of water, your tank is a closed ecosystem.

That means your fish are trapped in their environment and can't move elsewhere if they feel threatened or stressed. It also means that wastewater can accumulate rapidly with no way to get rid of it except for human intervention.

This mixture of behavioral and waste factors means that an overstocked tank can be a big issue. If your fish are stressed, they are much more likely to get health problems. Overstocking can be one of the most common causes of fish death.

Factors Influencing Stocking Levels

planted fish tank
Source: Reddit

It’s important to know that stocking your fish tank isn’t an exact science. There are multitude of factors that can affect whether or not your tank will be overstocked.

Tank Size –If you wanted a tank with a large variety of colorful fish, then you’ll most likely need to buy something quite large. Larger tanks not only offer a greater level of water volume, but this also means diluting of toxins and more space for bacteria to thrive. Smaller tanks are more vulnerable to fluctuations, and therefore more vulnerable to overstocking.

Species of Fish – Different species have different space requirements and behavior patterns. Some can also be more territorial and aggressive, which can be made worse by overstocking. Make sure to research the needs of each fish before you set up your fish tank.

Filtration and Maintenance – You need to ensure that your filter is able to process the volume of water in the tank and the amount of waste that will be produced. Regular maintenance will also be required, especially for smaller tanks.

Compatibility – Even a tank with few fish could be overstocked if the species aren’t compatible. Some thrive in community setups such as tetras, where others, such as bettas, are happy to be left on their own.

Calculating Stocking Levels

overstocked fish tank
Source: Cichlid Forum

There are no strict rules for stocking levels. However, there are some good general rules to follow.

Inch per Gallon Rule – This is the most famous rule and dictates that you should only have one inch of fish for every gallon of water. As you may notice, that rule is highly convenient and doesn’t account for the varying needs of fish.

If you’re a little bit over an inch per gallon, it’s usually not going to be an issue. It’s an oversimplified approach but it’s a good starting point for whether or not your tank is overstocked.

Surface Area Rule – Another common rule is that you should have one inch of fish for every 12 square inches of surface area. This can be a decent rule for traditional tanks but doesn’t take into account depth or uniquely shaped tanks.

Bioload Consideration – Along with the number of fish, you also want to consider bioload. Some fish produce more waste than others. For example, goldfish have a high bio load, so they are more vulnerable to being overstocked.

Stop and Think About Stocking

overstocked tetra tank
Source: YouTube

Now we know how many fish you should put in a tank. Next, it's important to carefully consider what fish you want to place inside it. If you already have one fish in mind, look at what other species would be the greatest tankmates for them.

This is even more important if you have a small tank, as waste can quickly build up. In addition to having tank mates that are non-aggressive, you can also consider species that will live in different areas of your tank.

For example, you have bottom feeders such as catfish that will stay close to the substrate. Barbs are usually happiest near the mid to lower level of the tank, and danios prefer to be near the surface. Taking this into consideration can be a great way to have a well-stocked tank without the stress of overcrowding.

Work out what fish you want and how they'll interact, and keep roughly to the gallon-per-inch rule. If you're unsure, then your local fish store is usually more than happy to give advice on the best way to stock your tank.

How To Cope with Overstocking

overstocked cloudy tank
Source: FishLore

We also appreciate that you may be reading this article and your fears have come true: you have an overstocked tank. If so, what should you do about it?

Calculate the Problem – Note down all your fish, their rough length, and add it all together. If you’re looking at two inches per gallon, for example, then that’s probably a big issue.

Change the Tank – The obvious step is to upgrade your tank to a larger size. If that’s not possible, then consider buying a second tank to split the load.

Rehome Some Fish – If expanding or adding a tank isn’t possible, look to rehome the fish. Asking on social media is a good place to start, or you can take them back to a fish store.

Provide Hiding Spaces – If your tank is only slightly overcrowded, then look at ways to reduce stress. Providing hiding places is essential and these can be provided by plants or ornaments.

Monitor Water Quality – More fish means more waste. Check the water more often than usual to ensure toxins aren’t spiking.

The Bottom Line

bad tank stocking
Source: Reddit

The number one rule with stocking fish is, well, there is no rule. It depends on all the factors above, and all the guides we've looked at have flaws. Let's take the one-inch-per-gallon rule as an example.

Having 12 one-inch fish in a 6-gallon tank isn’t the end of the world. However, putting one 6-inch fish into a 6-gallon tank would be cruel, as they wouldn’t have space to swim. It’s important to use common sense and research the fish to ensure you’re meeting their needs.

It’s best to start off small and monitor your parameters. Once your tank is established, you can then look at adding more fish to the tank. As long as your fish are happy and stress-free, you can have peace of mind that you’re giving them a supportive ecosystem.

Kenny Jarvis
By Kenny Jarvis

Kenny is a passionate animal lover who finds joy in the diverse world of pets. He frequently embarks on zoo adventures with his children, immersing himself in the wonders of nature. At home, Kenny tends to a much-loved aquarium, nurturing a thriving underwater ecosystem. Through his passion for writing, he aims to help fellow animal lovers create happy and healthy environments for their pets.