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 several ways to minimize 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. The aquarium filter is a key location where beneficial bacteria can grow. Make sure your aquarium has adequate filtration. 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. For a happy, healthy aquarium, you can follow these instructions. If you need to get started, consider purchasing used filter media, or live nitrifying bacterium to jump start the cycle.

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. Fast-growing plants such as stem plants or floating plants can remove nitrogen waste faster than slower-growing plants.

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

An aquarium water test kit is necessary to ensure that your fish are happy and healthy. It will determine if the nitrogen waste levels are at 0 parts per million (parts/million) ammonia, 40 parts/million nitrite and 0 parts/million (parts/million) nitrate. 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 would you like to make water changes? You can do it once a week, every two weeks or once a month. You will have more fish if you change the water more often.

Fish Food

There are many fish foods that are not all 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 such as Xtreme Nano and frozen foods, on the other hand, don’t produce as much waste. This is why they are recommended as “clean” food.

Even if you are only feeding high-quality fish food, keep in mind that more fish will poop. Plus, some fish are very “messy” because they tend to leave leftover scraps, which will rot in the water if not removed. 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. Ten 1-inch Tetras, for example, have a different body volume than 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 grow to eight inches (20 cm) in size. As such, a 20-gallon tank is 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). Angelfish would do well in a 29-gallon aquarium measuring 18 inches (46cm) high.

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. Some fish like zebra danios are only 2 inches (5 cm) long but are very active and need more swimming room. Others fish might be bigger ambush predators, which don’t move as much and thus require less space. Some species prefer to live in small groups of 6-10 fish. This can have an impact 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

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. 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 are known to be aggressive and will often stay at the top, if they see other fish swimming close by. 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 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 suppose you have a 20 gallon aquarium filled with live plants. You want to add community fish.

1. Figure out which species of fish and invertebrates you want to add and find out if they are all compatible with each other in terms of temperament, size, aggression level, living conditions, and similar diet. 2. Choose a set frequency at which you plan to do water changes. 3. Add your favorite species first. It is best to add the minimum recommended amount of fish to a schooling fish first, in order for the aquarium’s ability 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 at understocking your fish aquarium. Most aquarium ecosystems are composed of a large number of plants, but fewer fish. It’s similar to how a forest has many trees and few deer.

Remember that your fish tank is a living ecosystem and will change over time. Some species breed quite readily and the population boom may increase the waste load, so you may need to remove fish to compensate. Also, healthy plants grow over time which reduces the waste load and decreases the swimming area. The aggression level of any fish added to a tank may be affected by their addition. Over time, you will become more proficient at keeping fish tanks safely and without causing harm to the residents. To learn more about our blog posts, videos and new product launches, sign up to our weekly enewsletter.