How Big Will Aquarium Fish Get In Their Natural Environment? When aquarium fish are released into the wild, their environment changes on so many levels. As a result, they will also eventually adopt new behaviors as a survival mechanism. The transition from an artificially made environment to a natural environment can go in a positive (thrive) or negative (die) direction, too. 

A Must-Read: What Are The Best Aluminum Fishing Boats?

Fish released into a natural environment will grow as large as their genetic disposition, food availability, and water conditions, among other factors, allow them.

The Size Of The Body Of Water Isn’t The Be-all And End-all

There’s a common misconception that fish will grow depending on the size of their aquarium. But there’s a wide range of factors, both uncontrollable and controllable, which influence the final size of fish in a tank. Indeed, the size of the tank can be considered as a secondary factor in the growth of fish.

On the other hand, fish can only grow as much as their genes allow them. Even if the aquarium or the natural body of water has the perfect conditions for its growth, it will become only as big as its genes dictate it. 

Fish that are naturally small due to their genes will remain small even when released into the wild. Examples include the Indonesian dwarf fish, pygmy hatchet fish, and the Gulf Coast pygmy sunfish. It also applies to naturally large aquarium fish like the giant gourami,t  dinema catfish, and iridescent shark.

But it must also be noted that fish are intermediate growers in the sense that many of them can grow throughout their lives. Take the domestic goldfish (Carassius auratus) that can become oversized given the right food and water conditions, including a large body of water. But it will not grow, say, as large as a tiger shark because Mother Nature doesn’t design it to so.

On the other hand, fish will grow as much as their food resources, water temperature, and size, and place in the food chain allow them. In an aquarium, these are controllable factors since you. The owner can easily change them.

You can transfer them to a larger fish tank, feed them a varied diet, and remove their natural predators, among others. Your fish will then grow as large as you want them to but within their genetic limits. You have, in other words, the ability to ensure that your pet fish will thrive, often regardless of the size of their tank.

But in the wild, these controllable factors become uncontrollable factors! Every aspect is up to the whim of Mother Nature. As such, fish previously kept in an aquarium can either thrive or die within a short time after release, depending on its adaptation mechanism. 

But it is not always true that the fish in tanks and aquariums, particularly in hobby ones, are smaller than the fish that thrive in their natural environment. Goldfish are an excellent example since these are usually small in tanks but can grow as large as a full-sized carp in their natural environment. 

The bottom line here: While the size of the tank has a bearing on the size of an aquarium fish, it isn’t the be-all and end-all of size. Itis also true for aquarium fish released into the wild, or a natural body of water.

Contrary to popular opinion, it will not grow automatically larger just because it was transferred to a larger environment. A wide range of factors, from food and water conditions to predator presence, will affect its growth. 

These Factors Matter More Than The Size Of The Body Of Water

Let’s take a closer look at the primary factors where fish growth, size-wise, is concerned. Keep in mind that these are interrelated factors such that the causes of one will have an impact on another, even become the cause. For example, an inadequate diet in the wild can be caused by incorrect water conditions for algae to thrive.

The Parents And Genes Of The Fish

As we mentioned earlier, the size of a fish will be significantly influenced by its genetic composition. Yes, there will be variations in the size of fish in a single species, but these are usually slight. For example, an Indonesian super dwarf fish may grow to as much as 0.41 inches, but it won’t grow to an inch even with perfect natural conditions.

There’s also the impact of line-breeding of aquarium fish on their growth rate. Linebreeding is the selective breeding of fish for their desired characteristics, usually by the selection of mates within closely related lines. The fish species commonly line-bred are tilapia, guppies, and goldfish.

While line breeding has its merits, it has also resulted in weaker breeding populations. Fancy goldfish, primarily the mass-produced ones, show weak development and poor body shape. Farmed guppies have a higher risk of disease as well as generally unhealthy.

When poor quality aquarium fish are released to the wild, they are less likely to thrive, much less grow in size. They are already at a disadvantage because of their poor health. Even if they managed to breed in the wild, their offspring would still be weaker because of their genes. 

Healthy aquarium fish, in contrast, will most likely survive in the wild, especially with the right food and water conditions. They will also likely grow to their maximum size within the natural limits of their species.

The Stressors Present In The Environment

Fish also have emotions, as ridiculous as it may sound! They react to the stressors and stimulants in their environment, whether artificial or natural, in several ways. They can decrease their food consumption, movements, and even their breeding cycle in response to these stressors.

The stressors can be changes in the water temperature and turbidity, in their diet, and the size of their habitat. The last factor is possible even in a natural environment, such as from a drain to the open sea. There’s also the matter of overcrowding in a habitat, the introduction of predators, and the presence of toxins, such as in polluted waters.

Fish have several reactionary responses to these stressors, and their use of these responses will determine their survival. Among these responses is homeostasis, an automatic process in living organisms that reengineers their physical, chemical, and internal conditions to achieve a steady-state. In short, homeostasis allows the fish to counteract the adverse effects caused by their environmental stressors.

If the fish achieves successful homeostasis, then it lives. But if it doesn’t for one reason or another, it’s likely to suffer from stunted growth, perhaps even prematurely die.

The process of homeostasis also affects the growth hormone, a type of protein with vital roles in the regulation of major physiological processes. These include metabolism of carbohydrates, growth of soft tissues and bones, and appetite as well as immune and reproduction functions. While the process of growth itself is a complex process requiring the interaction of numerous factors, the growth hormones are at the center of it.

But it isn’t just the growth hormone in a fish that affects its growth, size-wise! The hormones secreted by other fish in the habitat will also affect its size, and it’s a factor in both artificial and natural environments.

An excellent example is a common goldfish (Carassius auratus), a favorite in aquariums, which produces powerful growth-inhibiting hormones into the water. These hormones include gamma-aminobutyric acid (GABA) and somatostatin, both of which restrict the size of other fish in the habitat.

It means that when the common goldfish are released into the wild, they can quickly become larger than its size in the aquarium. They not only secrete growth-limiting hormones, but they can also be quite aggressive and, thus, get the lion’s share of the food. They can then grow larger than when they were in an aquarium and, in some cases, become an invasive species in local waterways.

The Opportunity For More Movement

Fish must have sufficient room for physical activity that, in turn, is vital for their proper growth and muscle development. But fish in aquariums have limited space to swim around and, thus, they don’t have the opportunity for active swimming. Unless the water movement is increased, they aren’t wont to engage in physical activity for fitness purposes.

How Big Will Aquarium Fish Get In Their Natural Environment?

The water movement in aquariums can be increased through powerheads like the Freesea Aquarium Wave Maker or spray bars like the Eheim AEH4004310 Spray Bar Set. But even with these devices, there’s still the matter of limited space for the fish to swim in.

How Big Will Aquarium Fish Get In Their Natural Environment?

In a natural environment, fish have more room for engaging in physical activity. The river waters and streams are usually fast-moving, while lakes and seas have currents and waves. These water movements allow fish to engage in fitness activities that contribute to their muscle development and physical health.

Healthy fish in their natural environment will then have higher chances for growth. Plus, their muscle mass and tone contribute to their survival in a world where the big fish eat the little fish. 

The Water Conditions Where The Fish Lives

Water comes in several forms, such as soft and hard, acidic and alkaline, marine and freshwater, to name a few. Freshwater fish will thrive in freshwater while saltwater fish will grow in the open seas and oceans. There are only a few species of aquarium-suitable fish that can live in both conditions, such as the American eel.

So, if you release goldfish into the sea, they will not survive because they are freshwater fish. Conversely, clownfish and firefish will not grow to their maximum size, much less survive, if they are released into freshwater like rivers and streams.

The Availability And Variety Of Food

Of course, fish need sufficient amounts of food for proper growth! But it isn’t just the availability of food that matters but the type of food, too, since the latter is vital in getting the full range of nutrients. In an aquarium, it’s easy to change the amount and type of fish food according to the species and age of the fish. 

But in the natural environment, the food conditions are different. There may be too little food or too little variety depending on the body of water, as is the case for polluted rivers or lakes. Where the fish may have enjoyed a steady and stable supply of food in the aquarium, their food becomes less plentiful, less varied.

And with less food comes malnutrition! Malnourished fish will not grow as their genetic disposition allows them, even when all the other factors are near-perfect. The effects of malnourishment range from loss of appetite, poor food conversion, and fat collection in the organs to muscular wasting, lethargy, and poor growth. 

All of these things provide an impact on the growth and development of fish released into their natural environment. The interplay of these factors will also determine whether the released fish will thrive or die.

Dangers Of Releasing Aquarium Fish Into The Wild

But we don’t recommend releasing aquarium fish into the wild for whatever reason. Many species of fish, particularly the goldfish, can become invasive and threaten the local flora and fauna.

The goldfish is of particular concern for many reasons. First, their feces can encourage the development of algae blooms that disrupt the ecosystem in lakes and ponds. Second, they can grow so large and grow in numbers so quickly that they can adversely affect the food chain. They may even promptly demolish the food sources upon which the local fish species depend upon.

Many areas in the United States are also being besieged by the issues related to aquarium dumping. The state and local governments can spend hundreds of thousands of dollars on water rehabilitation. It is right in the West Medical Lake in Washington and Lake Tahoe in California.

What then can you do instead of aquarium dumping if you can’t care for your pet fish?

  • Find a member of the family or friend who can them in
  • Contact your local animal shelters and pet shops since some of them may take in fish
  • Join a fish club and give your fish to willing persons
  • Ask your veterinarian for humane fish disposal methods if they are ill

If your issue is about proper care for your pet fish, you should read guidebooks like Freshwater Aquariums for Dummies.

How Big Will Aquarium Fish Get In Their Natural Environment?


To conclude, fish that have previously lived in aquariums and tanks will only grow as large as their new natural environment allows them. They will not become super-sized fish, even in the best conditions, because of their genetic dispositions.  But they can grow bigger than their tank-bound counterparts, and that may or may not be a problem for the rest of the fish.