How Many Fish Per Gallon (A Complete Answer)

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You did it! Your fish tank is finally cycled and ready to be filled with every species you’ve ever dreamed of keeping. You can’t wait to go to your local fish store and buy 100 fish for your 10 gallon fish tank.

But wait, maybe you should rethink that plan. Not all fish can be kept together and there is a limit as to how many you can keep in your tank, no matter how big it might be.

Here’s how to figure out how many fish per gallon your freshwater or saltwater tank can support.

Key Takeaways

  • There is no set rule for how many fish per gallon you can keep in your freshwater or saltwater aquarium.
  • Many factors, like experience level, tank shape, tank setup, fish species, and water quality will determine the right number for your tank.
  • Stocking a saltwater aquarium is much different than stocking a freshwater setup and more consideration is needed.

Freshwater Stocking

Whether you’re stocking a fish tank for the first time or trying to add a little more biodiversity to an established aquarium, you need to know how many fish per gallon is right for your system. You may have heard of the one inch of fish per gallon rule, especially if keeping a freshwater tank. This means that every inch that your fish is expected to grow in length requires an additional gallon of water.

We’re here to say that the one inch of fish per gallon rule is obsolete and was never true.

In reality, this golden stocking number changes from tank to tank and there is no correct answer. In fact, there are many factors that go into determining how many fish can comfortably and safely fit in your aquarium.

There is a big difference between how many fish can be kept in a freshwater aquarium as opposed to a saltwater aquarium although many of the determining factors are the same. To understand how many fish you can keep in your aquarium, you will need to understand your experience level, tank shape, tank setup, fish species, and water quality.

Experience Level

Before you even think about filling your fish tank with water, you need to take a realistic look at your capabilities as a fish owner. For beginners, a larger aquarium is generally easier to keep stable than a small aquarium while also allowing plenty of space for fish. On the other hand, experienced hobbyists can easily fit multiple schooling species and feature species into small tanks without any problems.

An experienced hobbyist can overfill an aquarium, while inexperienced hobbyists may struggle with maintaining a well-understocked aquarium. Why is this?

Behind fishkeeping is a ton of science with even more trial and error. Unfortunately, some lessons can only be learned by doing. This experience lets hobbyists make judgments about their tank’s abilities that determine how successful they are and ultimately allow for more fish.

Tank Shape

Believe it or not, tank shape influences how many fish you can keep in your aquarium over tank setup. This is because many tropical fish prefer horizontal swimming space as opposed to vertical swimming space; one of the exceptions to this rule might be freshwater angelfish (Pterophyllum scalare)1.

About a decade ago, bowfront and hexagonal aquariums became very popular. However, hobbyists quickly found out that they are impractical for the natural behavior of fish wanting to swim long distances. It should also be mentioned that it was difficult to find appropriately-fitting equipment.

One of the best examples of how big of a difference the tank shape can make is with stocking between a 20 gallon long tank and a 20 gallon high tank.

20 gallon tanks are very desirable for their stretched-out, yet confined space. This is one of the best tank sizes available that can house an assortment of fish, invertebrates, and plants. This is in direct contrast to a 20 gallon high tank which is usually too short for keeping more than one kind of tropical schooling fish.

In general, a long fish tank allows for more fish than a tall fish tank.

Tank Setup

That isn’t to say tank setup isn’t important, though. The more items you put into an aquarium, the less space your fish have to move around. That is, unless, you add live plants.

There are many different ways to set up a fish tank. For freshwater aquariums, the main setups are split between artificial and natural setups. Artificial setups include aquarium-safe decorations while natural setups include live plants, rocks, and driftwood.

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How does your choice in decorations affect which fish you can keep though?

Every species of fish comes from a different environment with conditions that are unique to that ecosystem. While many fish were born and raised in the aquarium hobby, recreating their natural habitat in an aquarium setting is ideal. Depending on the species, some fish like wide open swimming areas while others enjoy a tank filled to the brim with plants; some fish even prefer nothing at all in their tank.

Filtration & plants

No matter which tank setup you go with, it is always recommended to keep live plants if possible. Live plants offer natural biological filtration as they filter and use harmful compounds, like ammonia, nitrite, and nitrate. Having a balance between live plants and the amount of bioload created in the aquarium allows for naturally safe conditions, and thus more fish as long as more plants are then added.

Otherwise, additional filtration can make up for having a large amount of fish in the aquarium. For especially dirty species, some hobbyists have filtration systems that are the same size or larger than the display tank! Filter media is necessary surface area for beneficial bacteria to grow and process waste; the more filter media you have, the more potential bacteria you will have.

Installing a large filtration system with plenty of media can work to process ammonia and nitrite, allowing more biological room for fish and less physical tank maintenance.

Tank Maintenance

If you can’t keep aquarium plants and are limited with filtration, then there are ways to have more fish without them. The main way to have more fish without plants or a big filtration system is by performing regular maintenance. This method is not recommended for the average hobbyist as missing even a single water change on an overstocked tank can be fatal.

Depending on how many fish you have, you may need to perform daily water changes. These water changes can vary in percentage, but some of the larger predatory species may require up to 75% of water changed daily; the more waste created, the bigger and more frequent the water changes need to be.

Keep in mind that the amount of tank maintenance you need to perform doesn’t necessarily depend on how many fish are in the tank, but rather on the species, the filtration system, what they eat, and the overall bioload created.

Fish Species

To answer how many fish you can keep in your aquarium, you need to look at the species being kept. The truth is that there is no correct answer to how many fish can be kept per gallon as every tank is different.

In general, larger fish need a larger tank and smaller fish need a smaller tank. But if we look at the tank size requirements for a betta fish versus a tetra, we’ll see all of the considerations that must go into making sure our fish are comfortable.

The smallest tank a betta can be kept in is 3 gallons. The smallest tank a neon tetra can be kept in is 10 gallons. On average, betta fish grow to be about one to two inches bigger than a tetra. So why do they need so much more space?

Tetras are schooling fish that need to be kept with their own species. When you buy one neon tetra, you’re actually buying at least six due to their schooling behavior. Neon tetras are generally more fast-moving and active than betta fish, so they also need more space to freely swim. Though bettas are larger fish, they excel in small spaces that reflect their natural habitat.

However, if wanting to put your betta fish with other schooling fish, then you need a much larger tank, like a 10 gallon. This is because you need to account for the betta’s aggression.

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Aggression and territory establishment are the main reasons why some fish need so much space. Many fish in the freshwater hobby are peaceful and suitable for a community tank, meaning that you can keep a lot in one tank. However, aggressive species, like cichlids, need plenty of room to establish and maintain their territories and dominance. This leads to fewer fish in a bigger tank.

In addition to aggression, fish waste, leftover food, and overall bioload also need to be considered. Bigger fish eat more and create more waste, but they’re not the only ones. Some fish are notoriously messy, like small platies (Xiphophorus sp.) that have very active bowels. Again, the messiness of a fish will vary from species to species regardless of their size.

Lastly, you need to consider the maximum size of your fish. Many fish are sold as juveniles in the aquarium hobby, but should be expected to grow larger as they mature; for some fish, this is a couple of inches, while others can turn into a couple feet in difference. One good example of this is the fancy goldfish (Carassius auratus).

It is common to find a fancy goldfish for sale under 4 inches when in reality, their adult size might actually be a foot in length! Considering the maximum size of your fish might make your tank seem empty at first, but give your fish some time to grow into it.

Water Quality

No matter your experience level, tank setup, or species kept, water quality is the most important aspect of safely stocking your aquarium. Every aquarium must be cycled and established with good bacteria that are able to process toxic compounds that would otherwise kill your fish. If your tank has not completed the nitrogen cycle, then not even one fish can be safely kept.

But what does it mean to have good aquarium water?

Good aquarium water doesn’t mean anything but having safe water parameters that make your fish happy. Ideally, this should read as 0 ppm ammonia, 0 ppm nitrite, and under 40 ppm nitrate with a neutral 7.0 pH and a tropical water temperature in a freshwater setting.

In general, a greater number of fish equals a greater amount of waste entering the system, which can increase toxic compounds, like ammonia and nitrite. This is why many beginner hobbyists lose all their fish in the first few days of having them due to a low number of beneficial bacteria in an overstocked tank.

However, even a large aquarium with little fish can have water quality issues. This mainly comes down to not having adequate filtration or regular maintenance but can also be due to leaving a dead fish in the aquarium or overfeeding. If you neglect your tank, then conditions can quickly become unsafe.

It’s important to keep in mind that while you don’t want an excess of harmful compounds in your aquarium, many hobbyists have kept fish in water parameters outside of these ideal standards for decades.

Saltwater Stocking

Stocking a freshwater aquarium is much easier than stocking a saltwater one. Saltwater systems are more delicate regarding tank setup, fish species, and water quality. There is no rule book to stocking when it comes to keeping fish in the saltwater hobby.

For instance, you need to decide which tank setup you want to have: a reef or fish only with live rock (FOWLR). If setting up a FOWLR tank, then you need to pick whether you want to keep reef species or predatory species. Predatory saltwater fish require a lot of room due to their aggression and food requirements, which in turn, creates a lot of waste. This means that a large tank is required with above-average filtration and a very particular stocking list.

Reef species are easier to find tank mates for, but still challenging to get right due to increased aggression, territoriality, and special considerations that come along with most saltwater species. The order in which the fish are added can also determine the long-term success of the setup.


How many fish can live in a 1 gallon tank?

None! There are no available species of fish that are suitable for a 1 gallon fish tank. The smallest aquarium size ever recommended for keeping fish is 2.5 gallons which will comfortably fit a betta under experienced hands.

How many fish can you put in 2 gallons?

Again, none! A half gallon makes a ton of difference in the pico world. The minimum tank size recommended for any fish is 2.5 gallons.

How many fish can you put in a 5-gallon?

5 gallon aquariums don’t leave much space for fish. A betta fish tank is the most ideal setup, though freshwater hobbyists have had success keeping specific kinds of killifish and minnows.A 5 gallon saltwater aquarium can comfortably fit some species of goby and invertebrates.

What fish can be in a 10 gallon tank?

Many fish, both freshwater and marine, can be kept in a 10 gallon tank. These options include popular tetras, rasboras, and livebearers as well as clownfish and gobies.

How many tetras can you put in a 10 gallon tank?

Ideally, six to eight tetras should be kept in a 10 gallon tank to form a complete school. Some hobbyists have had luck keeping two schools of different tetras in a 10 gallon tank, though this is recommended for experienced hobbyists as this would be considered a crowded tank.

How many fish can I put in a 10 gallon tank with a betta?

This largely depends on the personality of the betta and how many other fish it will tolerate. In general, a peaceful betta can be safely kept with an active species of schooling fish.


There is no rule for how many fish you can keep per gallon of water for freshwater or saltwater setups. Many factors, including experience level, tank shape, tank setup, fish species, and water quality determine the right number of fish for your exact aquarium. In our opinion, it’s always better to understock the aquarium than to deal with water quality issues or aggression problems in the future.

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