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. A high level of waste can cause water quality problems and even fish 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.


Beneficial bacteria naturally grows in our fish tanks and is responsible for consuming toxic waste compounds like ammonia and eventually converting them into 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. This article will help you decide which fish tank filter is best 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

Another method to remove toxic nitrogen waste is through live aquarium plants. They eat the nitrogen compounds and use them as food. The aquarium can take more fish if there are more plants. In general, fast-growing plants like stem plants and floating plants remove nitrogen waste more rapidly than slow-growing plants.

A lush forest of aquatic plants can absorb large amounts of toxic waste made from fish poo, leftover foods, and other organics.

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

All fish foods are not 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. Plus, some fish are very “messy” because they tend to leave leftover scraps, which will rot in the water if not removed. 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 10-inch oscars have 10 times the body volume of a 10-inch tetra. It is important to factor in the swimming space if you want to keep larger fish.

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 allow the goldfish to swim approximately 30 inches (76cm) back and forth, and 12 inches (30cm) to turn comfortably around. An angelfish’s body is vertically oriented and has a length of 6-inches (15 cm) as well as a height 8-inches (8.8 inches). 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. Some fish like zebra danios are only 2 inches (5 cm) long but are very active and need more swimming room. Some fish are larger ambush predators and require more space. Additionally, some species are schooling fish that prefer to live with at least 6-10 fish in a group. Consider the impact this has on the overall waste burden. The maximum size of the fish is also important. Most fish are sold as juveniles at the fish store and may double or triple in size by the time they reach maturity, so make sure your tank has enough swimming space for their final adult form.

#3 Aggression Level

A last category to keep in mind is 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. To allow weaker fish to escape or hide from dominant fish, you may need to add a lot of decorations and plants.

Another example is a betta fish living in a community tank. 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. If this is the case, it may be a good idea to pick tank mates that swim in both the middle and bottom layers of your aquarium. This will ensure that your betta fish doesn’t get in their way.

How to determine the right stocking level

If your aquarium has been cycled (i.e. it contains beneficial bacteria and/or plants growing), then the best way to determine how many fish you can add is to measure the nitrate levels and make sure they are below 40ppm. Let’s suppose you have a 20 gallon aquarium filled with live plants. You want to add 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. You can choose a frequency at which water changes will be performed. 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. Measure the nitrate level each week for 2 to 3 weeks in a row. Once you are certain that the water quality stays high and you can consistently keep the nitrate level below 40 ppm, 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. This slow and systematic method of adding fish to your aquarium gives the beneficial bacteria colonies time to react and grow accordingly.

Aim for understocking your fish tank. Aquarium ecosystems that are stable have a lot more plants than fish. This is similar to a forest with fewer deer.

Remember that your fish tank is a living ecosystem and will change over time. Some species reproduce very quickly and you might need to remove fish to make up the difference. Healthy plants also grow over time, which decreases the waste load but cuts into the available swimming space. 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. Subscribe to our weekly email newsletter to receive the latest blog posts, videos, product announcements, and more.