How many Fish can i Put in A Fish Tank?

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 …

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 you can reduce your waste load.


The natural growth of beneficial bacteria in fish tanks is essential. It consumes toxic waste compounds like ammonia and converts them into less toxic compounds, such as 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.

You won’t have enough beneficial bacteria to clean the aquarium if you only just purchased the filter. To create a healthy, happy environment for your fish, follow our aquarium cycling instructions. You might also consider buying used filter media or live nitrifying bacteria in order 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 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 the beneficial bacteria and live plants are not able to consume the waste compounds quickly enough, then you must manually “take out the trash” yourself by removing some of the old aquarium water and adding fresh water with dechlorinator. What frequency do you plan to water change? Once a week, once every two weeks, or even once a month? You will have more fish if you change the water more often.

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 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 your fish are of high quality, it is important to remember that aquariums produce more poo than they consume. Fish can also be messy, leaving behind leftover food that could rot in their aquarium. 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 applies to small community fish of approximately 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 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 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). 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.

If you are unsure of the minimum size tank for the fish you want to keep, it is best to research the size that works best. Even though they are just two inches (5 cm), some fish, such as the zebra danios require more 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. 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

Last, be aware of the aggression level of your fish. 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.

Another example is a betta fish living in a community tank. Bettas tend to hang out near the top of the tank, and can become aggressive when other fish swim close to 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 Determine the Right Stocking Level

Assuming your aquarium is already cycled (e.g., has a healthy amount of beneficial bacteria and/or growing plants), the easiest way to figure out how many fish you can add to an aquarium is by measuring the nitrate level and making sure it stays below 40 ppm. Let’s say you have a 20-gallon aquarium with live plants and you want to start adding 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. 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. Repeat Steps 3-4 for adding any more species to the tank.

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 methodical method of adding new fish also gives the beneficial bacteria colony time to react and multiply 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.

Your fish tank is a living system and will continue to change. Some species reproduce very quickly and you might need to remove fish to make up the difference. Also, healthy plants grow over time which reduces the waste load and decreases 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.