How many Fish can i Put in A Fish Tank?

How Many Fish Can I Put in a Fish Tank?

We get many questions about this question, but it is one of our most challenging. 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. The water quality can drop if the waste levels are high, which can cause fish to become sick or even die. 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. An aquarium filter is one of the main locations where beneficial bacteria likes to grow, so make sure you have adequate filtration that is appropriate for your aquarium size. Read this article to learn about which fish tank filter is right for you.

You won’t have enough beneficial bacteria to clean the aquarium if you only just purchased the filter. To create a healthy, happy environment for your fish, follow our aquarium cycling instructions. You might also consider buying used filter media or live nitrifying bacteria in order to jump-start the cycle.

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. The more plants you have, the more fish the aquarium can handle. Fast-growing plants, such as stem and floating plants, remove nitrogen waste quicker than slow-growing ones.

A dense forest of aquatic plants that are actively growing can absorb large quantities of toxic waste from fish poo and leftover food.

Tank maintenance

In order to keep your fish happy and healthy, use an aquarium water test kit to make sure the nitrogen waste levels measure at 0 ppm (parts per million) ammonia, 0 ppm nitrite, and less than 40 ppm nitrate. If beneficial bacteria or live plants cannot quickly consume the waste compounds, then you need to manually “take out” the trash by removing some old aquarium water and adding new water with dechlorinator. How often do you want to commit to doing water changes? You can do it once a week, every two weeks or once a month. The more frequently you change water in your aquarium, the more fish you will be able to keep.

Fish Food

Not all fish foods are created equal. Low-quality foods often break apart easily and contain a lot of filler ingredients that are not digestible, which create more waste. 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. Fish can also be messy, leaving behind leftover food that could rot in their aquarium. If you have a messy eater like an oscar, try getting some scavengers that will eagerly clean up after it.

Swimming Space

Beginners were often advised that 1 inch of fish be kept for every 1 gal of water. 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. It is important to factor in the swimming space if you want to keep larger fish.

Fancy goldfish can grow up to 8 inches (20cm) in length. Therefore, a 20-gallon aquarium is recommended as the minimum tank. These dimensions allow the fish to swim comfortably back and forth for approximately 30 inches (76 cm). Angelfish are vertically oriented, with a length of 6 inches (15 cm) and a height of 8 inches (8.8 in). An angelfish-friendly aquarium would be a 29-gallon tank that is 18 inches (46cm tall) in volume.

Adult Angelfish can reach 8 inches in height. Make sure you have enough vertical space in your fish tank to accommodate them.

Research the minimum tank size for each fish you plan to keep, and go with the largest recommended size if possible. Zebra danios, for example, are just 2 inches (5 cm) in length but require more space. Some fish are larger ambush predators and require more 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. Finally, look at the maximum size of the fish. The majority of fish sold at fish stores are juveniles. They may grow up to three times their size when they reach adulthood. Make sure you have enough space in your tank for them to swim.

#3 Aggression Level

The aggression level of your fish is another important factor to consider. African cichlids require that you have more fish and less space. This will ensure that there is no one fish in the area that can defend and establish its territory. In order to allow the weaker fish to escape and hide from the dominant, you might need to add many decorations and plants.

Another example is the betta fish who lives in a tank with other fish. 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. You may choose to have tank mates who swim in the middle or bottom layers of the aquarium, and that will keep your betta fish safe.

How to Determine the Right Stocking Level

Assuming your aquarium is already cycled (e.g., has a healthy amount of beneficial bacteria and/or growing plants), the easiest way to figure out how many fish you can add to an aquarium is by measuring the nitrate level and making sure it stays below 40 ppm. Let’s say you have a 20-gallon aquarium with live plants and you want to start adding community fish:

1. Find out what species of fish and invertebrates are compatible with one another and determine if they have similar temperaments, sizes, aggression levels, living conditions, and similar diets. 2. Choose a set frequency at which you plan to do water changes. 3. Add your favorite species first. If it is a schooling fish, consider adding the minimum recommended number at first to make sure the aquarium can handle the waste load. 4. For 2 to 3 consecutive weeks, measure the nitrate levels each week. After you have established that water quality is good and the nitrate levels are below 40 ppm each week, you can add your next favorite species. 5. For adding more species to your tank, repeat steps 3-4.

While many novice aquarists prefer to purchase large numbers of fish quickly, it is best to initially stockpile your aquarium and add fish later. The beneficial bacteria colony will also benefit from this slow, methodical approach to adding fish.

Aim to understock your fish tank. 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.

Your fish tank is a living system and will continue to change. Some species breed quite readily and the population boom may increase the waste load, so you may need to remove fish to compensate. The waste load is decreased by healthy plants that grow over time. However, this can reduce the swimming area. The aggression level of any fish added to a tank may be affected by their addition. 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. If you’re interested in leveling up as an aquarium hobbyist, sign up for our weekly e-newsletter to learn about our latest blog posts, videos, and product releases.