Do goldfish really grow to the size of their tank?

A common theory about fish is that their size is somehow based on the volume of water they inhabit, such as a tank, and to a new owner this might seem true.

But fish are malleable. They are “intermediate growers” and can continue to pick up mass throughout their life.

Sometimes though, given poor living conditions, a fish will look as though it’s not growing at all. This is probably where the myth started, because the fish we see – at home and in aquariums – are typically smaller than fish that thrive in the wild.

So what causes these fish to dwarf?

First off, the size of an enclosure is not the root of your shrunken fish. Living in an enclosure simply means facing pitfalls that occur more rarely in a natural habitat.

Fish can be dwarfed by a number of factors. They are gelatinous, frail, cold-blooded, emotional vertebrates that stress at the slightest hitch. There are a few uncontrollable factors limiting growth, as well as a slew that can be controlled.

What can’t be controlled is fish age, genetics and species. These dictate the point where growth rates naturally taper off, and do not stunt young fish.

But when a fish encounters stressors such as changes in temperature, poor diet, confinement, overcrowding or polluted water, things starts to fluctuate. Not only do these cause obvious physical setbacks, such as hunger leading to lower metabolism, they cause cellular complications too.

An encounter with one or more of these stressors is followed by a response – a reactionary tool that stirs up hormones to help fish get through stress.

Fish do this through an automatic process that reengineers internal biology to counteract any ill-effects caused by their surroundings, otherwise referred to as homeostasis.

In a natural environment, stress affects fish briefly and their hormones quickly reset. But a fish confined in a closed space will go on stressing until its owner takes notice.

Among these fluctuating elements is growth hormone, which plays a big part in the regulation of major physiological processes. These include appetite, skeletal and soft tissue growth, carbohydrate metabolism, reproduction and immune functions.

If homeostasis is successful, the fish lives; if unsuccessful, death or tragedy follows.

Experts conclude this trick is an evolutionary advantage that helps fish survive if they get stuck in a small body of water. But for tank-based fish it’s also a step backwards.

The common goldfish, or Carassius auratus, is a prime example of this. They produce potent growth-inhibiting hormones such as gamma-aminobutyric acid (GABA), and can even secrete pheromones like somatostatin to limit the size of rival fish in their waters.

Alongside these pheromones that fester in unchanged water and dwarf C. auratus, the fish has to find ample food while limiting the toxicity of the water, which is filled with immune system-suppressing nitrates from its waste that can lead to life-threatening changes in pH levels.

Other fish do not produce hormones as potent, but have been linked to the production of growth-inhibiting hormones such as GABA.

In a natural environment, stress affects fish briefly and their hormones quickly reset. But a fish confined in a closed space will go on stressing until its owner takes notice.

The stress response becomes a maladaptive process in this case, and leads to the misconception that “the fish only grew to the size of my tank”. The fish has indeed only grown to the size of the tank because after passing that size, it died.

In most cases a stunted fish can recuperate by nursing the water quality of its enclosure or by tweaking temperature and diet.