Do goldfish grow to the size of their tank?

I received a fairly lengthy question yesterday from the UK. To summarise the question, it was about whether goldfish only grow to the size of the tank (the full question/s follow this answer).
Growth in fish, like other organisms, are influenced by many factors including nutrition and hormonal influences. The one thing though, the people mean when they talk about talk about goldfish growing to the size of its tank is to do with their regulation of growth hormones. 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! And in this way, it is also a survival mechanism whereby it will not outgrow its pond!
Other fish do not produce such potent hormones and this is why they can outgrow the aquarium they live in. These fish tend to be “big fish in a big pond”. Their survival strategy is to get as big as they can to avoid being eaten by someone else. The barramundi and Murray cod are great examples of such fish.
Fish wastes are generally not ideal for fish to live in. They would have anti-nutritional effects and nitrates are known to suppress the immune system. If conditions are not optimal, fish will not thrive and will not grow.

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Yours sincerely,Dr Richmond Loh
BSc BVMS MPhil (Vet Path) MANZCVS (Aquatics & Pathobiology) DipPM

Veterinarian / Adjunct Lecturer Murdoch University / Secretary Aquatic Animal Health Chapter ANZCVS
The Fish Vet, Perth, Western Australia. Mobile Veterinary Service for fish and other aquatic creatures.
Ph: +61 (0)421 822 383
On 27/02/2012, at 18:54,
Dear Richmond Loh,
I came across your information on under the ‘Fish Vets Listing’ section and I was wondering if you could possibly help me.

I have recently been investigating and trying to find out the truth behind the issue of Goldfish growth, how it is affected through stunting and what it is in particular that causes stunting to begin with.

There are many rumours, opinions and myths floating around, which makes it hard to discern the facts from the fiction. I was wondering if you could help me with the understanding of this topic. Or know of any papers/research that could help point me in the right direction.

At the moment, the main things I come across when I speak to others, is that stunting is caused due to the fish being kept in small aquaria, poor water quality or under feeding.
While I can appreciate these are all valid points, how do these factors cause the growth to cease? Through looking around I have begun to see people say, that due to hormones a goldfish can continue to grow, regardless of it’s tank size, if it has good water quality. It makes me wonder, what hormones they are referring to and the biology behind this? Along with what counts as small aquaria? When is a tank, too small for a goldfish?

Another interesting rumour I’ve heard is that when a goldfish is stunted, it’s body stops growing, but it’s internal organs do not? Is this true?
I am also curious, at how a goldfish is affected, if it’s stunted at different sizes. For example is a goldfish stunted at 8 inches, much healthier than one stunted as a juvenile?

If you could provide any answers, or further help, I would be extremely grateful.

Thank you for your time,
May I wish you the best for the future,

Ms A. Taylor


10 thoughts on “Do goldfish grow to the size of their tank?

  1. Surely it does.When i had the same doubts i created a 2 nos of 1000 ltrs tank. tank 1 was 10L*2h*2w ft and tank 2 was 4L*5h*2wft.Both had the same type of filter & pumps(1000ltrs gravel based filter,3000lts/hr pump,1000ltrs/h pump for spraying water over the surface). tank 1 had horizontal water current while tank 2 with greater depth had more top to down flow.both tanks where stocked with feeder gold bought form the same shop and same tank. were fed same amount of feed two times a day. no aquatic plants or fertilization was done.water parameters were maintained well for both the tanks. After 4 months the fishes in tank 1 out grew the fishes in tank 2 by 3 folds in length and weight. No mortalities were noted,both the stocks looked happy and disease free.The more they swim the more is the feed converted to flesh.In tank 1 fish had a great surface area to swim while in tank 2 the surface area for swimming was cut in half. gold fishes(unless they are big enough) cant handle the water pressure of depths more than 2 ft. the camera in tank 2 hardly recorded fish swimming in depths more than 2 ft thus limiting their swimming area. so when all other parameters are good fishes remain healthy and the swimming area for the fishes sure has a great impact on fish growth.


  2. I note with disappointment that Richard Loh (Phd or assumed title?) does not really address all the OP’s questions which are of real interest to most serious goldfish keepers.


  3. Hello! Thanks for this article.
    Can you tell me other species or families which have this “hormones issue” like goldfish?
    For example, Cichlidae? Anabantidae? Guppies and poecilide in general?


      1. I personally doubt that hormones play any significant part in goldfish keeping. My observations are that they all grow, & variations in growth rate are more marked within species, than between a small & large fish in the same aquarium environment.
        Furthermore the recommended tank water change routines, would be expected to significantly dilute the concentration of “hormones” if hormones did exist.


      2. From my reading and observations, hormones are powerful substances capable of altering moods, activity levels and even sexual development. There are many lakes where fish populations have become all-female due to oestrogenic compounds.


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