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. The water quality can drop if the waste levels are high, which can cause fish to become sick or even die. You should not have so many fish that they cause illness by the waste they create in your aquarium. There are many ways to reduce waste.

Filtration

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 one of the most important places where beneficial bacteria thrives. It is essential that you have sufficient filtration to suit your aquarium size. This article will help you decide which fish tank filter is best for you.

You won’t have enough beneficial bacteria to clean the aquarium if you only just purchased the filter. Follow these aquarium-cycling instructions to create a healthy and happy environment for your fish.

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. In general, faster-growing plants like stem and floating plants are able to remove nitrogen waste quicker than slower-growing ones.

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

Use an aquarium water tester kit to check the levels of nitrogen in your tank to make sure that they are below 40ppm nitrate and ammonia. If beneficial bacteria or live plants cannot quickly consume the waste compounds, you can manually “take out” the trash by removing some old aquarium water and adding dechlorinator to fresh water. What frequency do you plan to water change? Do you want to change water once a week? Once every two weeks? Or once a monthly? You will have more fish if you change the water more often.

Fish Food

Not all fish foods are created equal. Low-quality foods can often be difficult to break apart and include a lot more filler ingredients than are digestible. This creates more waste. High-quality foods, such as Xtreme Nano frozen food and frozen food, are the opposite. They produce less waste.

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

In the past, it was often recommended to beginners that you can keep 1 inch of fish for every 1 gallon of water. This rule of thumb mainly applies to small community fish that are approximately 1-3 inches (2-7 cm) in size. A 10-inch oscar has a larger body volume than a 10-inch Tetra. You should consider how much swimming space you have if you intend to keep larger fish.

A fancy goldfish can reach 8 inches (20 cm), so a 20 gallon-long aquarium is often recommended. These dimensions give the goldfish about 30 inches (76 cm) to swim back and forth, as well as 12 inches (30 cm) to comfortably turn around. However, if you get an angelfish, its body is vertically oriented with a 6-inch (15 cm) length and 8-inch height. 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. Even though they are only two inches (5 cm), some fish, such as the zebra danios require more room. 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. 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

Remember to also consider the aggression level your fish have. 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 for weaker fishes to escape from the dominant ones, it is possible to use a lot more decorations and plants.

A betta fish in a community aquarium 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. In that case, you may want to choose tank mates that swim in the middle and bottom layers of the aquarium and will mostly stay out of your betta fish’s way.

How to Determine the Right Stocking Level

If your aquarium has been cycled (e.g. it contains beneficial bacteria and/or plants growing), you can easily figure out how many fish you are allowed to add. To do this, measure the nitrate levels and make sure they stay 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. Set a time frame for 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. Each week, measure the nitrate concentration for 2 to 3 days. Once water quality has improved and the nitrate level is below 40ppm, then you can add the next species. 5. You can add more species to the aquarium by repeating steps 3-4.

Many beginners aquarists want to purchase large quantities of fish immediately. However, it’s better to overstock your aquarium first and then get more fish later. This slow and systematic method of adding fish to your aquarium gives your beneficial bacteria colonies time to react and grow accordingly.

Aim for understocking your fish tank. A stable aquarium ecosystem will have many plants but few fish. This is much like a forest that has many trees but not many deer.

Keep in mind that your fish tank will be a living ecosystem. It 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 addition of any new fish may change the aggression level in the tank. As you get more experience with fish keeping, your ability to safely maintain a larger fish tank and not harm its inhabitants will also change. To learn more about our blog posts, videos and new product launches, sign up to our weekly enewsletter.