Hmmmm. One of my favorite topics that has to be revisited at least semi annually."Apparently the reason this myth is still around is because even though the fish stop growing on the outside of their body, they continue to grow on the inside."
No, not quite true, but in a manner of fashion a good stretch answer. Its not really a matter of organs continuing to grow as it is that they stop developing properly or stop growing. Thus, as Karen mentioned....stunting. This generally occurs when a fish is kept in extreme conditions such as the goldfish bowl scenario. Most times it is simply a matter of non-growth (not to be confused with true stunting).
I am going to repost some info that was presented from a formal badmans chat presentation back in Dec 2004:
Due to technical difficulties there were several missing portions of the information Russ was trying to present. Below is the text of his notes in full:
Good evening. Many of you know that my involvement in aqauriology has been both as a hobbyists and in the wholesale and retail end of the pet (fish) business. For tonight's topic, I chose to use the phrase 'non growth' rather than 'stunting'. I would like to reserve the word stunt(ing) for future reference in the presentation because I hate polictical correctness. Not thats there is anything wrong with it...lol
There will most likely be some questions after the presentation, but before getting started, I would like employ a disclaimer. Plese try to not ask a question about, "I have a three year old XYZ fish that is supposed to grow to 10 inches." "It is now only 5 inches." "If I place it in a 100 gal tank next week, will it then grow to it's capable size?" Well, I'll be straight forward on something like that. I don't know! Nobody will know. However, the information presented here will hopefully help determine the odds of it doing so or not doing so.
I should inject at this point that for discussion purposes here, tonight, I'd like to stick with freshwater, tropicals and what is generally known as 'ornamental' fish. And, to include common brackish, without getting into marine husbantry and care of marine fishes. Although there are same basic similarities as far as growth factors, lets keep this simple.
Tonight you will have to put this information into perspective. Unless you are more than just a hobby breeder, and have several rows of aquariums in a 'fish room', your perspective should be as a hobbyist.
As a hobbysits, you will strive to acheive a small piece of an ecological system with selected fish species and plant species, while maintaining an appropriate environment for the tank inhabitants to live and thrive. I'm not discounting those who do have several rows of tanks, but are not 'hobby breeders'. Theres that political correctness that just bit me.
Lets start by attempting to define what 'stunting' is. In it's most simple terms, stunting is merely a 'check' in developement. As far as our fish is concerned,, this can mean either a temporary or permanently stoppage in developement, or growth. Before delving into some of the 'root causes', it should be known that a 'stunted' fish is not the same as a fish that fails to regrow certain portions of it's body. Failure to regrow an eye or a complete major appendage, would not constitute stunting anymore than our inability to regrow a lost finger.
Most of the research I have explored and experienced all point to ideal "requisites" and "energy allocation" in determining whether a fish is stunted or is capable of regrowth. Functional requisites and resulting energy allocation is controlled by the fish's physiology.
Not withstanding vital organs that may have been damaged through disease, combat, or congenital defects, physical operations of organs is regulated in accordance with vital importance to sustaining life.
There are also environmental factors that contribute to the 'ideal' climate that a fish requires to not only function, but to grow and reproduce. Keep in mind that fish have developed in regions throughout the world for millions of years. They have adapted, thrived, and reproduced in these regions because they are in a favorable situation that contributes to sustaining the fish's requisits. And they have evolved in those regions.
Also,(this is important to remember) please keep this point in mind during this discussion as it relates to growth and reproduction: "The systems responsible for the basic metabolic functions necessary for life, such as the nervous system, the respiratory system, and the systems that control osmoregulation, have a higher priority for energy than growth or reproduction."
At this time, I like to divide further discussion into two main categories that would affect growth and nongrowth of our fishes.
ENVIRONMENTAL and PHYSIOLOGICAL
Above, I mentioned requisites and energy allocation in determining whether a fish is stunted or capable of regrowth. While 'requisites' encompass both environmental and physilogical factors, energy allocation and functional requisites of the fish would fall into the physiological category when we are dealing with our individual aquariums.
99.9% of the environmental factors are controlled by the hobbyist and can be further broken down into other groupings that could include:
RESOURCE LIMITATION FACTORS and ECOLOGICAL FACTORS
To provide your fish with an ideal requisite that it can live and thrive (grow and reproduce), the hobbyist must provide an conducive environment in order for the fish to be physiologically sound. Most of us have heard the saying that one should purchase the largest tank they can afford? Actually, one should purchase the largest tank they can afford to maintain. This is a 'resource limitation' This is also basically controlled by the hobbyist.
Tank size, stand to support the tank, canopy and light, heater, thermometer, test kits, water and electric bills, purchasing budget, and last, but certainly, not least, commitment time towords the fish's diet and the tank's maintenance. These are resource limitations factors that must be considered. Providing the type of water that goes into the aquarium, the type of structure (plants, gravel, and rocks) that go into the tank, and how they are arranged, actual fish selection, diet, and how much or how often the fish are feed, are all ecological factors that should be considered.
At this point you are probably wondering how all this ties into growth or non-growth. Take a breath. We're getting to it.
Environmental issues such as the amount of toxins, dissolved organic compounds, total dissolved solids, biological oxygen demand, temperature, and photo periods can impact the general health and well being of our fish. Environmental such as resource limitations and ecological factors play a signifficant role in the fish's physical health and impact on it's basic physiology. Providing favoralbe resources and employing sound ecological principles will lead to the relief of the main cause of non-growth and stunting of a fish. This is 'stress'
Stress, in itself, like stuning, is an effect, and not the root cause. These root causes have already been identified above. That is that environmental and physiological factors can lead to stress, which in-turn can/will lead to non-growth and stunting. So, how is stress defined? For simplicity sake, lets say it is the physiological response from the fish to adapt to a stimulus.
These responses to stress situations are part of a series of physiological reactions called "the general adaptation syndrome" or adaptive factors. This sydrome is divided into three phases: (1) the alarm reaction, when hormones are released (2) a stage of resistance, during which adaptation occurs (3) and, if the fish cannot adapt, a stage of exhaustion, which can be followed by death.
ACUTE and CHRONIC Stress:
The initial response to an acute stress is called the "fight or flight" response. As an example, when a fish is pursued by a predator or fish net, the stress response results in a instantaneous increase in available energy, which may allow the fish to escape the predator or elude the fish net. Other energy-requiring functions such as osmoregulation are temporarly shut down so as much energy as possible can be focused on escape and evasion. This is a short-term measure to produce large amounts of energy to deal with an emergency situation.
Most acute stresses are short-term.
So, the fish either escapes (or deals with the emergency) and the stress is releived or the fish gets caught or can't deal with the emergency. In the wild, there is plenty of room to escape both predators and unfavorable water conditions. In captivity, fish are often subjected to long perriods of stress from many different causes. A very common cause of death in captivity as a result of being constantly stressed is osmotic shock. Remember, a fish cannot correct the osmotic problems caused by acute (short-term) stress.
Stress also supresses aspects of the immune response. A frequent results is the outbreak of disease after fish have been subjected to unfavorable physiological conditions.
A fish exposed to chronic stress can eith adapt to the stress, or fail to compensate and die. Even if a fish does compensate, its performance capacity will be reduced during the period of compensation. Many cases, even after. For example, fish can adapt to rather wide ranges of temperatures, but within this broard temperature range there is a preferred range in which the fish will grow and perform best. The further from this range, the poorer the fish's performance.
Under conditions of little or no stress, there is an energy surplus that can be put into growth and reproduction. However, the further conditons are from the preferred range or "requisite", the greater amounts of energy the fish must use to make the proper physiological adjustments. A prime example of chronic stress is poor water quality. If the water quality and other factors causing the problem are corrected, growth generally improves, and the fish may begin to reproduce.
There is a lot that can be said about the diet and feeding of fish that will also impact greatly in determining growth. I've opted for a general overview rather than typing complex formulas and/or equations.
Foods loaded with vitamins cannot be overly stressed. Vitamins are organic compounds that serve as catalysts for many biochemical reatons in body tissues. *** Deficiency of almost any vitamin can result in retarded growth and increase susceptability to disease.
So, can a fish that has been kept in a smaller than recomended tank for an extended period of time, be latter transferred to or into a favorable environment, and the fish's requisites are met, resume growth? Yes, it is very possibe, as long as vital organs are not damaged from the result of disease or chronic stress.
My simple take has been this all along.........keep fishes from the same regions and continent in water that you can most easily provide for, feed a balanced diet, and maintain 'requiste' water permameters. Your fish should grow to it's capacity.
References in books listing a certain pH or temperature, hardness, etc., was not published for the heck of it. Most very good authors of those books and references did a great amount of research over the years to determine most of the known fish requisites. It would not be unwise to follow their advice once in a while. There are also a great many folks on this site with enough skills and experience to help other folks with info on their fish's requisites for growth and reproduction.
I bet if I just posted the link to this on our site, less than half the folks reading this would follow it...That is why I just thought it better to copy and paste the cliff notes