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Home > General > Do Goldfish Grow to the Size of Their Tank? Vet Approved Info

Do Goldfish Grow to the Size of Their Tank? Vet Approved Info

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Reviewed & Fact-Checked By

Dr. Luqman Javed

Veterinarian, DVM

The information is current and up-to-date in accordance with the latest veterinarian research.

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The battle between people who believe goldfish require large tanks and people who believe they can happily live in small tanks constantly rages, with both sides bringing arguments for or against to the table. One of the most common arguments you hear for keeping goldfish in smaller tanks is that goldfish won’t outgrow the size of their enclosure, which many people think sounds ridiculous and like something that is not at all based on science. After all, the size of a goldfish’s tank can’t impact their growth to that extent, can it? Let’s separate out the fact from the fiction about whether or not goldfish grow to the size of their tank.


Is It True?

In a strict sense, goldfish do not grow to the size of their tank.

There are actually multiple factors that influence the growth of goldfish, and while the size of the tank is technically not one from a scientific point of view, it is a very important one from a practical sense for most fish keepers. Water quality, nutrition, stress levels, and health status can also play big roles in how large a goldfish grows to or how small it remains.

To a certain extent, goldfish will grow to the size of their tank, but you have to understand the mechanism of action that causes this effect for there to be any chance of it happening in your tank.

Panda Moor Goldfish
Image By: hxdbzxy, Shutterstock

What Causes This to Happen?

Goldfish have two types of secretions that can impact maximum size in an enclosed environment. One is growth-inhibiting hormones, like somatostatin, and the other is the production of inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA). Hormones are secreted by goldfish into their environment, which they may then absorb systemically, stunting their growth. Goldfish are one of many freshwater fish that have the ability to do this.

What happens when goldfish excrete these hormones is that they begin to build up in the water. In the wild, this means that fish that are trapped in small ponds are living in a higher concentration of hormones and pheromones than those that are living in rivers or lakes.

Now, apply this knowledge to your aquarium. It’s a small enough environment to allow for the buildup of hormones and pheromones in the water, which, in theory, should mean that your goldfish won’t outgrow the tank, right? Well, not exactly. Why?

To understand how goldfish can outgrow their tank, it’s best to explore another form of fishkeeping.

Closed System Aquarium

The way most people house fish is in an aquarium, where a fixed volume of water is constantly circulating around a fish (via a filter) and only occasionally refreshed partially via a water change. This is also known as a closed system.

Every time you remove water from the tank, you’re removing hormones and pheromones with it, and you’re obviously not replacing them when you add new water to the tank. After a water change, the concentration may have decreased significantly or only slightly, depending on the size of the tank, the number of fish, when the last water change was performed, and the amount of old water removed from the tank.

Image Credit: dien, Shutterstock

Open System Aquarium

Another rather expensive but oddly interesting form of fish keeping is known as an open system. In this setup, fish are housed in an aquarium and the way they’re maintained is by ensuring that there’s a constant flow of new water from a safe source being added to the aquarium at all times. Concurrently, there’s a constant flow of water being removed from the aquarium at the same rate. The result is an aquarium that always holds the same volume of water, but never the same water, as it keeps refreshing itself from a safe source all the time.

This form of fish keeping requires a great deal of investment in terms of the initial setup, and is considerably expensive, as you need a constant supply of water. However, it is also fascinating as there’s no buildup of toxins in the water with this system. Such an aquarium doesn’t have to be cycled prior to adding fish in it. This setup of an aquarium essentially tries to replicate a fish in a stream.

With this form of fish keeping, any hormones that a fish produces are instantly removed from the water. Researchers have found that fish kept this way possess the ability to outgrow their environment. When housed in open system tubes, fish raised experimentally kept growing until they couldn’t move or turn in the tube. However, the tube itself didn’t restrict their growth. Therefore, to think fish will only grow to the size of their tank is technically incorrect.

However, given how the majority of fish are housed in closed systems, the tank’s size ends up playing a very important role in determining your fish’s size.

Will Allowing Hormones and Pheromones to Build Up Harm My Goldfish?

No, allowing these hormones to build up in the tank won’t directly harm your fish. However, what can and will harm your goldfish is poor water quality. Infrequent water changes allow for the buildup of waste products, especially nitrates, and the build-up of solid waste like poop and uneaten food. The more these things build up in your tank, the worse your water quality becomes and the more likely your goldfish is to experience negative health effects.

It is hypothetically possible to find a balance between few enough water changes to allow for hormones to become concentrated in the tank and frequent enough water changes to maintain good water quality, but there really is not an exact science to tell you how to do it. This is because no test exists to measure these hormones.

Your top priority should be maintaining water quality, so always make your tank cleaning and maintenance plans with this goal in mind.

goldfish plant_gunungkawi, Shutterstock
Image Credit: gunungkawi, Shutterstock

Will Stunting Growth Harm My Goldfish?

Currently, there is no definitive science that indicates that allowing your goldfish’s growth to be stunted is harmful to them. Some goldfish are naturally small and will stay small, even if you have them in a 200-gallon pond, while others may continue to grow, even in a tank that’s on the small side. Either way, these fish will grow and develop properly, barring underlying health conditions.

Some people are concerned that allowing growth to become stunted may allow for the external body to stop growing while the internal body continues to grow, leading to organ enlargement and failure. Scientists have not conclusively proved or disproved this theory as of yet.

However, from a humane point of view, there is no justifiable explanation for allowing your pet goldfish to be stunted. As explained previously, goldfish can outgrow small tanks, and therefore, if you observe your goldfish stunted in a small aquarium, it means that they were unfortunately exposed to stressors that caused the stunting in the first place.

Alternatively, it could also indicate that the nutritional value of the food offered to your goldfish wasn’t adequate for them. Either explanation has no reasonable justification.


Final Thoughts

Goldfish are far more interesting fish than they are often given credit for, and their ability to induce stunted growth in enclosed environments is fascinating.

Allowing your goldfish’s growth to become stunted isn’t cruel or proven to be dangerous as far as science has shown. However, science also hasn’t justified any reason for allowing this to happen in pet goldfish either, as it offers a pet goldfish no benefits. In addition, a stunted pet goldfish is usually the result of poor housing conditions or nutritional deficiencies and therefore, there is no logical reason to allow your goldfish to become stunted.

Featured Image Credit: Skumer, Shutterstock

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