How Many Fish Can I Put in a Fish Tank?

The most difficult and common question we receive is “How many fish do I need to keep 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.

Filtration

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. The aquarium filter is a key location where beneficial bacteria can grow. Make sure your aquarium has adequate filtration. 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

Live aquarium plants are another method of removing toxic nitrogen waste from the water because they consume the nitrogen compounds as food and use the nutrients to grow more 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. What frequency do you plan to water change? Once a week, once every two weeks, or even 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 food can break down quickly and have a lot filler ingredients that are difficult to digest, leading to more waste. High-quality food like Xtreme Nano pellets or frozen foods are the exact opposite. They don’t create as much waste and can be used as a substitute for conventional foods.

Even if you are only feeding high-quality fish food, keep in mind that more fish will poop. Fish can also be messy, leaving behind leftover food that could rot in their aquarium. 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 10-inch oscars have 10 times the body volume of a 10-inch tetra. If you plan to keep bigger fish, the amount of swimming room becomes an important factor to consider.

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). 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. 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. Some species prefer to live in small groups of 6-10 fish. This can have an impact on the overall waste load. Finally, look at the maximum size of the fish. 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

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. 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 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. 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 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 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. Choose a set frequency at which you plan to do water changes. 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.

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 for understocking your fish tank. 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 can breed very easily, so it is possible to have to remove fish in order to compensate. 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. To learn more about our blog posts, videos and new product launches, sign up to our weekly enewsletter.