How many Fish can i Put in A Fish Tank?

How Many Fish Can I Put in a Fish Tank?

One of the most common but hardest questions we get is “How many fish can I put in a 10-gallon tank? How about a 20-gallon aquarium? 55 gallons?” As you may guess, there is an infinite number of possible fish combinations for each aquarium size that we could recommend. To simplify things, let’s first understand the three factors that will most impact your fish stocking levels and then discuss our general guidelines for introducing the right number of fish to your aquarium.

#1 Waste Load

If you are not familiar with the aquarium nitrogen cycle, it explains that when fish eat food, they end up producing waste, and then beneficial bacteria and live plants help to break down those waste compounds. If the waste level builds up, the water quality goes down and can lead to fish illness or even death. Therefore, it is important that not to put so many fish in an aquarium that the waste they make causes them to get sick. There are many ways to reduce waste.


Our fish tanks naturally contain beneficial bacteria. This bacteria is responsible for the consumption of toxic waste compounds such as ammonia, and then converting them to less toxic compounds like Nitrate. A fish tank filter is the most important place for beneficial bacteria to grow. You should ensure that your aquarium is properly filtered. Read this article to learn about which fish tank filter is right for you.

After you have set up the aquarium and purchased the filter, the beneficial bacteria that will be needed to help your fish grow and process their wastes and maintain the water’s cleanliness won’t exist. Follow our aquarium cycling instructions to prepare a thriving, healthy environment for your fish, and consider getting some used filter media or buying live nitrifying bacteria to jump-start the cycling process.

Aquarium Plants

Aquarium plants can also remove harmful nitrogen waste from water. Live aquarium plants eat nitrogen compounds as food, and then use the nutrients to grow new leaves. An aquarium can hold more fish if it has more plants. Fast-growing plants, such as stem and floating plants, remove nitrogen waste quicker than slow-growing ones.

A lush forest of actively growing aquatic plants is capable of absorbing large amounts of toxic waste produced by fish poo, leftover food, and other excess organics.

Tank Maintenance

To keep your fish happy, you should use an aquarium water testing kit. This will check that the nitrogen levels are below 40 ppm. If your fish cannot consume the waste compounds fast enough, you will need to manually remove some aquarium water and add fresh water with dechlorinator. How often do you want to commit to doing water changes? Do you want to change water once a week? Once every two weeks? Or once a monthly? You will have more fish if you change the water more often.

Fish Food

All fish foods are not created equal. Low-quality fish foods are often difficult to digest and break down easily. High-quality foods like Xtreme Nano pellets and frozen foods are the opposite and do not create as much waste, which is why we recommend them as “clean” foods.

Even if you only feed high-quality fish foods, remember that the more food you feed the aquarium (whether you have lots of little fish or one big fish), the more poop is produced. Some fish can be very messy and leave behind food scraps that will eventually rot in the aquarium. Try getting some scavengers to help you clean up after a messy fish like an oscar.

Swimming Space

It was common for beginners to keep one inch of fish per gallon of water in the past. This rule is only applicable to small fish in the community that are between 1-3 inches (2-7cm), in size. For example, ten 1-inch tetras do not have the same body volume as one 10-inch oscar. If you plan to keep bigger fish, the amount of swimming room becomes an important factor to consider.

A fancy goldfish can potentially grow to 8 inches (20 cm) in length, so a 20-gallon long aquarium is often recommended as the minimum tank size. These dimensions give the goldfish about 30 inches (76 cm) to swim back and forth, as well as 12 inches (30 cm) to comfortably turn around. However, if you get an angelfish, its body is vertically oriented with a 6-inch (15 cm) length and 8-inch height. Angelfish would do well in a 29-gallon aquarium measuring 18 inches (46cm) high.

Adult angelfish may eventually reach an 8-inch height, so make sure your fish tank has the vertical height to accommodate them.

Research the minimum tank size for each fish you plan to keep, and go with the largest recommended size if possible. Even though they are just two inches (5 cm), some fish, such as the zebra danios require more room. Others fish might be bigger ambush predators, which don’t move as much and thus require less space. Plus, some species are schooling fish and prefer to live in groups of at least 6 to 10 fish, so consider the impact that has on the overall waste load. The maximum size of the fish is also important. A majority of fish are sold as juveniles in a fish shop. Fish can double or triple their size by the time that they reach maturity so ensure your tank has sufficient space to accommodate them.

#3 Aggression Level

Last, be aware of the aggression level of your fish. With African cichlids, the key is to add more fish and decrease the swimming space so that no single fish has the room to establish and defend its own territory. In order to allow the weaker fish to escape and hide from the dominant, you might need to add many decorations and plants.

A betta fish that lives in a community tank is another example. Bettas often hang out at the top of the tank and may get aggressive if other fish are swimming near the surface in their territory. In that case, you may want to choose tank mates that swim in the middle and bottom layers of the aquarium and will mostly stay out of your betta fish’s way.

How to determine the right stocking level

Assuming that your aquarium is cycled (e.g. has a healthy level of beneficial bacteria or growing plants), it is easy to calculate how many fish can be added to your aquarium. To do this, measure the nitrate level. Let’s say you have a 20-gallon aquarium with live plants and you want to start adding community fish:

1. Determine which fish and invertebrates species you wish to add, and then determine if they are compatible in terms of temperament, size and aggression. Also, consider similar living conditions and a similar diet. 2. You can choose a frequency at which water changes will be performed. 3. Add your favorite species first. To ensure that the aquarium can handle the waste load, you might consider adding the minimum number of schooling fish to your aquarium. 4. For 2 to 3 consecutive weeks, measure the nitrate levels each week. Once water quality has improved and you can maintain a nitrate level of less than 40 ppm, then you can add the next species. 5. Repeat Steps 3-4 for adding any more species to the tank.

Although many aquarists love to purchase large quantities of fish at once, it is better to start small and then add more fish as you progress. This slow and systematic method of adding fish to your aquarium gives the beneficial bacteria colonies time to react and grow accordingly.

Aim at understocking your fish aquarium. The most stable aquarium ecosystems usually contain a lot of plants and fewer fish, much like how a forest is full of trees with not many deer in comparison.

Keep in mind that your fish tank will be a living ecosystem. It will change over time. Some species can breed very easily, so it is possible to have to remove fish in order to compensate. Also, healthy plants grow over time which reduces the waste load and decreases the swimming area. The addition of any new fish may change the aggression level in the tank. You too will change and become a more experienced fish keeper over time, capable of safely keeping a more overstocked fish tank without harming its residents. To learn more about our blog posts, videos and new product launches, sign up to our weekly enewsletter.