How many Fish can i Put in A Fish Tank?

How Many Fish Can I Put in a Fish Tank?

We get many questions about this question, but it is one of our most challenging. 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. It is important to not allow fish to become sick from the waste they produce in an aquarium. There are several ways to minimize waste load:


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. An aquarium filter is one of the main locations where beneficial bacteria likes to grow, so make sure you have adequate filtration that is appropriate for your aquarium size. This article will help you decide which fish tank filter is best for you.

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

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. An aquarium can hold more fish if it has 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 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? 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

There are many fish foods that are not all created equal. Low-quality fish foods are often difficult to digest and break down easily. 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. Try getting some scavengers to help you clean up after a messy fish like an oscar.

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. 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.

Fancy goldfish can grow up to 8 inches (20cm) in length. Therefore, a 20-gallon aquarium is recommended as the minimum tank. 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. Angelfish are vertically oriented, with a length of 6 inches (15 cm) and a height of 8 inches (8.8 in). 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.

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. 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. 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. The maximum size of the fish is also important. The majority of fish sold at fish stores are juveniles. They may grow up to three times their size when they reach adulthood. Make sure you have enough space in your tank for them to swim.

#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. You may need to add lots of decorations and plants (which also decreases swimming space) in order to break up the line of sight so that weaker fish can easily escape and hide from the dominant ones.

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. 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 suppose you have a 20 gallon aquarium filled with live plants. 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. Choose a set frequency at which you plan to do 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. Every week, take the nitrate level and measure it for two to three weeks. 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. Repeat Steps 3-4 for adding any more species to the tank.

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 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 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 addition of any new fish may change the aggression level in the tank. 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. If you’re interested in leveling up as an aquarium hobbyist, sign up for our weekly e-newsletter to learn about our latest blog posts, videos, and product releases.