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 you can reduce your waste load.


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. Find out which fish tank filter you should use.

If you just bought the filter and set up your aquarium, there won’t be enough beneficial bacteria yet to process your fish’s waste and keep the water clean. 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. The aquarium can take more fish if there are more plants. Fast-growing plants, such as stem and floating plants, remove nitrogen waste quicker than slow-growing ones.

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 you water change in your aquarium, you can keep more fish.

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 your fish are of high quality, it is important to remember that aquariums produce more poo than they consume. Plus, some fish are very “messy” because they tend to leave leftover scraps, which will rot in the water if not removed. You might consider getting some scavengers if you have an oscar who is a messy eater.

Swimming Space

It was common for beginners to keep one inch of fish per gallon of water in the past. This rule applies to small community fish of approximately 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. You should consider how much swimming room you have if you intend to keep larger fish.

A fancy goldfish can grow to eight inches (20 cm) in size. As such, a 20-gallon tank is the minimum tank. 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). Therefore, a 29-gallon aquarium that is 18 inches (46 cm) tall would be more appropriate for angelfish.

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

Find out the minimum tank size you need for each fish that you wish to keep and then choose the recommended size. Some fish like zebra danios are only 2 inches (5 cm) long but are very active and need more swimming room. Other fish may be larger ambush predators that don’t move a lot and therefore 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. Also, consider the maximum size of your fish. 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

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.

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. 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 choose the correct 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 assume you have a 20-gallon tank with live plants. Now 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. 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. For adding more species to your tank, repeat steps 3-4.

Many beginner aquarists like to buy large amounts of fish all at once, but it’s always better to understock your aquarium at first and get more fish later if possible. This slow and systematic method of adding fish to your aquarium gives the beneficial bacteria colonies time to react and grow accordingly.

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. Any new fish can change the tank’s aggression. Over time, you will become more proficient at keeping fish tanks safely and without causing harm to the residents. Subscribe to our weekly email newsletter to receive the latest blog posts, videos, product announcements, and more.