How Many Fish Can I Put in a Fish Tank?

How Many Fish Can I Put in a Fish Tank?

6 min read

One of the most common but hardest questions we get is “How many fish can I put in a 10-gallon tank? What about a 20-gallon tank? 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. If the waste level builds up, the water quality goes down and can lead to fish illness or even 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. 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. Read this article to learn about which fish tank filter is right 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. The more plants you have, the more fish the aquarium can handle. In general, fast-growing plants like stem plants and floating plants remove nitrogen waste more rapidly than slow-growing plants.

gold barbs in planted tank

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

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. 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

Not all fish foods are 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 like Xtreme Nano pellets and frozen foods are the opposite and do not create as much waste, which is why we recommend them as “clean” foods.

Even if you only feed high-quality fish foods, remember that the more food you feed the aquarium (whether you have lots of little fish or one big fish), the more poop is produced. 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.

#2 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. For example, ten 1-inch tetras do not have the same body volume as 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 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. Therefore, a 29-gallon aquarium that is 18 inches (46 cm) tall would be more appropriate for angelfish.

blue zebra angelfish in community tank

Adult angelfish may eventually reach an 8-inch height, so make sure your fish tank has the vertical height 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. Other fish may be larger ambush predators that don’t move a lot and therefore require less 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. 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

A last category to keep in mind is 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. 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 a betta fish living in a community tank. 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

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. 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. 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. 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.

planted aquarium with white cloud mountain minnows

Aim to understock your fish tank. The most stable aquarium ecosystems usually contain a lot of plants and fewer fish, much like how a forest is full of trees with not many deer in comparison.

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. 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.