Do fish grow to the size of their tank?

Fish grow to the size of their tank.  At least this is what a lot of people believe.  There are a variety of factors that can limit a fish’s ability to grow properly but the size of the tank isn’t one of them.  A fish that has not grown to its full size is said to be ‘stunted’.  This means that a fish has not grown at the right speed, and to the right size, for its species.  This article will take you through some of the factors that can impact on a fish’s growth and explain why fish need an appropriately sized home in order to thrive.

Where has the idea that they grow to the size of their tank come from?
The idea that fish have special abilities to fit any given tank has developed out of a long term and widespread lack of understanding of their requirements.  As we learn more about their needs and requirements, and information becomes more widely available, we need to question some of the long held myths surrounding fishkeeping.

A lack of information and understanding has led to many fish being housed inadequately.  Some individual fish are robust enough to survive these conditions but their growth and health suffer as a result and they become stunted.  This has given the impression that they have ‘grown to the size of their tank’.  ‘Stunted’ means that a fish has not grown at the right speed, and to the right size, for its species.

Cold blooded animals are able to grow throughout their entire lives (Ettinger, date unknown).  If a fish kept in a small tank is moved to a larger one it may well have a growth spurt as it takes advantage of improved living conditions.  This can add to the illusion that it ‘grew to the size’ of the small tank and is now ‘growing to the size’ of the new, bigger one.

How big should a fish get?
All fish which are properly cared for should grow to the appropriate size for their species, their ‘natural genetic size’ as it were.  There will always be slight variations in size in any given population but the individuals should all be close to the average size for their species.

How do we know what is the ‘right’ size for any given species?
Studies of wild populations provide a lot of information about size.  Many breeders are also highly experienced in their specialist species and are able to feed their knowledge into the hobby.  There is a huge amount of research on the go at any given time by ichthyologists and others, this information all feeds into the hobby and adds to the already large amount of information available.  There are some excellent and reliable sources of information available to the hobbyist for their own research.  INJAF recommends the following websites for checking the potential size of different species:

What does a fish need in order to grow to its natural genetic size?
Fish need a variety of things in order to thrive.  To understand the right conditions for the species you keep it is important to research as much as possible, from reliable sources.  As a general rule fish need:

  • a big enough tank (see our article HERE on stocking your aquarium)
  • the right diet
  • enough of the right diet
  • appropriate water conditions
  • appropriate tank mates (bearing in mind that some species don’t like tank mates, in this case appropriate tank mates would be no tank mates)
  • appropriate tank decor

With these things in place your fish should grow to its full size and be nice and healthy.

What happens if they don’t have the right living conditions?
If something is wrong with a fish’s living conditions it could affect its ability to grow properly and be healthy.  The causes are usually intertwined with each other; one will often impact on, or cause, another.  Below we take you through some of the common causes of stunting.

A popular piece of information in the fishkeeping world is that you must not over-feed your fish.  This is true; you should research how much your fish should be eating and make sure you give them the right amount of the right food.

If you feed your fish far too much they may not be able to eat it all.  The uneaten food remains in the tank and rots down.  If the fish do manage to eat more than they should they may not be able to digest all the nutrients from it before it passes through their digestive systems.  This means that your fish will produce more waste.  If the filter bacteria are insufficient to process all the waste and/or ammonia from rotting food this will cause water quality problems which in turn will cause health problems for your fish.  This is why fishkeepers are advised not to overfeed.

In newly established tanks the advice is often to feed lightly whilst monitoring water quality for the first few days.  This advice is often aimed at new fishkeepers and is intended to help them avoid water quality problems when they first introduce fish after their fishless cycle.  As the tank set up and filter bacteria become established and the filter bacteria adjust to the amount of ammonia and nitrite needing to be processed the amount of food being offered can be increased to full rations.

Unfortunately underfeeding seems to have become a means of managing water quality in tanks which are too small for the species being kept.  Many keepers don’t realise that this is what they are doing.  They take advice not to overfeed and follow this advice carefully.  However, due to inexperience they don’t appreciate the bigger picture and often don’t realise their fish are suffering from malnutrition.  Goldfish, for example, eat a huge amount of food and are naturally very large fish.  It would not be possible to keep a goldfish in a small tank and feed it the right amount without experiencing poor water quality.

An underfed fish will not have enough nutrients for it to grow properly.  Its bones, muscles and internal organs will not develop properly.  It will be small and ‘weedy’ compared to healthy examples of its species.  It is unlikely to achieve its full life span.

Feeding your fish a lot of the wrong food can also have an impact on whether or not they grow and develop properly.  A carnivorous fish given food suitable for vegetarian fish will not get the right sort of nutrients for their needs and vice versa.  Species have evolved to eat the types of food available in their natural locations and this is what their keepers should try to replicate, as far as is possible and legal, in the home aquarium.

Norton Aquatics (online) state the following as the effects of malnutrition on fish which clearly a shows a link between malnutrition and poor growth:

Effects of Malnutrition on Fish.
The effects of malnutrition are listed by the mineral/vitamin and what condition is caused when the substance is lacking in the diet.

  • Vitamin A – Eye problems, loss of appetite, impaired growth, intramuscle and fin-base haemorrhage (internal bleeding), anaemia, dropsy, weakened gills.
  • Vitamin B (complex) – Loss of appetite, poor growth, fragile blood vessels, poor growth, anaemia, muscular wasting, convulsions, loss of equilibrium, cloudy eyes, poor vision, over-pigmentation, gasping and flaring of gill covers.
  • Vitamin C – Loss of appetite, over-pigmentation, eye haemorrhage, deformed cartilage and spine (commonly seen as a zig-zag shape from the dorsal to tail-fin – a ‘kink’ in the tail), intramuscular haemorrhage, anaemia, fragile blood vessels.
  • Vitamin D – Poor growth.
  • Vitamin E – Muscular wasting and poor growth.
  • Biotin – Loss of appetite, poor growth, muscular wasting, convulsions, intestinal lesions and convulsions.
  • Choline – Poor growth, poor food conversion, fat collection in liver, kidney and liver haemorrhage.
  • Folic Acid – Poor growth, lethargy, fragile fins, over-pigmentation and anaemia.
  • Inositol – Poor growth, distended stomach (dropsy – collection of fluid in the body cavity), skin lesions and increased stomach emptying time.
  • Niacin – Loss of appetite, rectal lesion, muscle spasm, skin haemorrhage, skin lesion and anaemia.

PH, GH and KH
PH: potential of hydrogen – a measure of acidity and alkalinity
GH: general hardness – a measure of the total minerals dissolved in the water
KH: carbonate hardness – a measure of the amount of carbonates dissolved in the water

Water comes in a variety of forms; soft, hard, acidic, alkaline etc.  Certain species are better adapted to certain types of water than others.  Fish which are adapted to living in hard water are unlikely to do well in soft water and vice versa.  This is especially true for wild caught fish, some of which often come from quite extreme conditions such as very low PH.  Captive bred fish are often more tolerant of ‘average’ PH/GH/KH water as they may have been bred and raised in this water for several generations.  This is not to say that there is no need to try and match captive bred species to their preferred water were they in the wild.  It’s simply that you may not need to match it to quite the extreme that the fish may experience in the wild.

Housing a fish in the wrong conditions can lead to health issues, for example, a species adapted to living in very hard water may suffer when placed in soft water.  The acidic water may damage cells and the lack of carbonates may affect the fish’s ability to maintain strong bones.  The fish will struggle to grow properly.  The immune system is likely to be compromised and the fish may pick up infections that a healthy individual would not.

Ammonia is the first stage of the nitrogen cycle.  Fish excrete ammonia through their waste and through their gills.  If ammonia is not removed from the water it can burn the fish’s membranes and damage gills.  A fish suffering ammonia burns may develop black patches on its body where the burns have damaged it.  If the gills are damaged it means the fish cannot breathe properly.  If the fish cannot get enough oxygen it will not develop properly as its body will not be properly ‘fuelled’.  Low level ammonia readings will not be enough to kill a fish but can adversely affect its long term health and development.

Nitrite is the second stage of the nitrogen cycle.  Nitrite binds with the fish’s blood and prevents the blood from carrying oxygen properly as it binds with the molecules that the oxygen should bind to.  Lack of oxygen being carried around the body will affect the development and effective functioning of the internal organs.  Nitrite has been shown to suppress growth in some species (Lewis & Morris, 1985).

Nitrate is the third stage of the nitrogen cycle.  Nitrate is usually only toxic to fish when it builds up to high levels.  In a suitably sized and well maintained aquarium this should not happen.  If nitrate does build up it can have a variety of detrimental effects on fish, including affecting growth rates (, 2008).

Hormones & Pheromones
Fish give off hormones and pheromones which in a closed system such as an aquarium build up in the water.  In a natural environment these would be diluted and/or washed away by the flow of a river or by rainfall in a pond.  Some can affect growth.  For example, Trudeau et al (2000) concluded that gamma aminobutyric acid (GABA) has been associated with inhibited growth in goldfish.  Goldfish are not the only species to produce this, therefore it is reasonable to link GABA to reduced growth in other species.

Matty (1985) also found that “many species of fish are capable of conditioning the surrounding water to produce an effect on other individuals.  The chemical cues thus produced facilitate or inhibit growth …” (Matty, 1985, p202).  Matty goes on to state that “the decrease in growth rate or stunting brought about by crowded conditions has been recognised for a long time” (Matty, 1985, p202).

Fish breeders have demonstrated that large frequent water changes can aid good growth as these dilute and/or remove some of the growth inhibiting hormones such as somatostatin.  Mark Brumell (PFK interview with Jeremy Gay, 2012) recommends an unlimited supply of fresh water for optimal growth in goldfish breeding.

Dr Richmond Loh recently answered a question posed by a Practical Fishkeeping forum member, Skwishee, on behalf of INJAF, regarding fish ‘growing to the size of their tank’.  Dr Loh stated that,

“Growth in fish, like other organisms, are influenced by many factors including nutrition and hormonal influences … Goldfish are one that produce growth inhibitory hormones (e.g. somatostatin) and in nature, it’s their way of reducing intraspecific competition by suppressing growth of other goldfish. This is a particularly useful survival mechanism especially if you’re a “big fish in a small pond”! In a tank situation, and if partial water changes are not performed regularly, this hormone can build up and suppress the goldfish itself!” (Loh, 2012).

The process described above by Dr Loh is intended to help the fish suppress the competition, in a closed environment the fish can end up suppressing itself which has a significant impact on its growth.

Some species produce an enzyme called thiaminase.  This is a digestive enzyme which destroys thiamine (vitamin B1).  Predator fish feeding on species which produce thiaminase can suffer from thiamin deficiency which can cause a variety of symptoms including poor growth (Lichtenberger, 2011).

Cortisol is a stress hormone that has been shown to cause reduced growth in various species of fish including goldfish by suppressing the appetite and reducing their ability to convert the food to energy (  Stress can come from overcrowding, poor water conditions, malnourishment, aggression from other tank mates, inappropriate décor such as lack of hiding places, to name but a few.

Muscle atrophy and lack of fitness
Fish housed in too small tanks will not be able to swim about as they should.  This means that their muscles will not develop properly so the fish will not have the muscle tone and bulk that it would if it were fully fit.

Parents and genes
As mentioned earlier, there will always be slight variations in size in any given species.  Line breeding has weakened the breeding population of certain species.  Farmed guppies for example can be prone to disease and are considered by some to be generally unhealthy which will have an impact on their growth (Hill, 2011).  Fancy goldfish have been line bred for generations by enthusiasts; some are bred very well and are strong and healthy.  However, some are ‘mass produced’ and many of these show poor body shapes and weak development.  Poor quality fish are unlikely to grow as well as good quality, healthy individuals.  If these poor quality fish are bred from their offspring will follow in their genetic footsteps.

There is a popular theory on the internet that the body stops growing but the internal organs do not.  We have not been able to find any evidence to support this but if you are aware of any please contact us at as we’d love to know more about this if indeed it does occur.

Summary and conclusion
The information provided above shows that tank size is not, per se, the limiting factor in fish growth.  It is a limiting factor but it is not the limiting factor.  Tank size can exacerbate other problems.  For example pollutants will build up more quickly in a smaller body of water, it is harder to feed a full diet and the fish will not have enough room to exercise properly.  Tank size does not cause these problems by itself.

The most common cause of stunting is a lack of understanding of a fish’s requirements resulting in a lack of appropriate care.  A stunted fish is not a healthy fish.


Lewis & Morris, 1985, Toxicity of Nitrite to Fish: A Review (pdf) available at (accessed May 2012)

Above link from Nitrite (web article) available at (accessed May 2012)

Norton Aquatics, Malnutrition – identifying nutritional deficiencies (online) available at (accessed May 2012), Is Nitrate Toxic? A Study of Nitrate Toxicity (online) available at (accessed May 2012)

Lichtenberger, M, 2011, Thiaminase and its role in predatory pet fish (and other piscivores) nutrition (online) available at (accessed May 2012)

Trudeau et al, 2000, The inhibitory affects of gamma aminobutyric acid (GABA) on growth hormone secretion in the goldfish are modulated by sex steroids (pdf) available at (accessed May 2012)

Matty, A J, 1985, Fish Endocrinology, Kent: Crookheim Ltd, from online source available on google books (accessed May 2012)

Gay, J, 2011, Best of British, Practical Fishkeeping, (online) available at (accessed May 2012)

Dr R Loh, 2012, Do goldfish grow to the size of their tank? (online) available at (accessed May 2012), Effects of cortisol on food intake, growth, and forebrain neuropeptide Y and corticotropin-releasing factor gene expression in goldfish (online) available at (accessed May 2012)

Hill, N, 2011 10 fish the trade might be better off without (online) available at (accessed May 2012)

Dr A Ettinger, Transcription Factors and Rod Photoreceptors available at (accessed May 2012)

And of course a big thank you to all those people on various forums who helped with sourcing references and providing information, your help is much appreciated.

Author: INJAF

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