Im reading up on building a small pond and one author stated that common goldfish grow to fit their containers. The example used was that a fish kept in a goldfish bowl would not change size as long as it stayed in the bowl, but would grow in size (45cms apparently) if its habitat let it. (Its a concern as I wanted to put the fish in the pond for the summer, and bring them into an aquarium in the winter. Not really possible if these fish grow like that). Thats a pretty big size increase if you ask me.
Is this true? Can a fish halt and restart its growth? Is it only goldfish? Or can other fish do this?
Don’t know a lot about it, but why do you assume that the fish has the ability to do anything? Seems to make more sense to assume that given the constraints of the fish’s size in relation to it’s environment it can get so much food and excersise and so forth, and thus grows or doesn’t without any will on it’s own part.
I don’t refer to the fish doing anything out of a sense of will. I understand that if its true its a reaction to its environment. What I am wondering is if it is something that fish do regularly. That is to say, if I have aquarium fish and dump them into a bigger environment, will they grow to huge size? Or is this simply an isolated instance for goldfish. Or doest it even happen at all? It just seems strange to me that those little goldfish one sees would all a sudden increase in size. I always assumed a species had a maximum size it would reach given sufficient food. Never thought that space would play so drastic a role.
OK – I read stuff into your OP that wasn’t there. Having settled that, I have absoutely no useful information to impart. 
We were told that when we bought our 2 small Irridescent Sharks. They quickly outgrew the 20 gallon tank we had them in, so we brought them back to the store and they bought them back. The guy told us there, that it was a myth. That fish will grow until they reach adulthood.
http://news.bbc.co.uk/hi/english/sci/tech/newsid_593000/593590.stm
This species does adjust growth. But I can say nothing about fish.
From what I vaguely recall from my aquarium-keeping days, the idea that fish only grow to fit their tank is false. It’s just something the salespeople tell the newbies.
More likely, they just die when they get too big. People frequenly buy replacement fish so this idea probably came to be an urban legend because no one can recall having a fish that grew to be the size of the tank.
Fish have a normal range of adult size. With proper care, they will grow to that range. With improper care, they will be smaller. If the normal adult size is too big for the tank system (e.g., not enough clean water cycling, too many other fish, etc.), then it will probably not survive.
I recall seeing many fish who were too big for their tanks (not physically larger than their tanks of course, but too big to be healthy in the given volume of water). Newbies usually pack in a huge number of fish into a tank because it’s fun, whereas it should be more like 1 fish per 5 gallons (for small freshwater fish) or 1 per 10-20 gallons (for small, hearty saltwater fish). There’s a rule of thumb for # of gallons per inch of fish…but I don’t recall it at the moment.
The concept of fish growing to the size of their tank can sometimes appear true, - though not always- (it depends on the species) but is actually a function of water quality and food.
You can stunt any fishes growth by starving them or keeping their water dirty. They just don’t grow well without proper nutrition and a healthy environment (think how well a baby would grow breathing an atmosphere full of swamp gas and car exhaust).
Fish do have general growth patterns, but many continue to grow if sexual maturity and energy devoted to gamete production don’t require all the juice and there’s a bit left over. Most are killed off in nature before they ever get to that stage, and those that don’t end up as trophy fish mounted on some fisherman’s wall.
Fish will grow to a size that matches their water quality and food source. You can litterally grow trout to the size and shape of bread loafs in containers no bigger than bread bags if you provide a constant flow of high-quality water at the right temperature and feed them all they’ll eat (high quality food too).
Then there is the matter of the fishes toughness and tolerance to stress and poor water quality and diet. Some species like tilapia will grow real big real fast in filthy water just grazing off the scum and algae that grows in it, while others will die overnight if the temperature, pH, hardness, tank mates, lighting, food, and stress levels aren’t all just right. So naturally “tough” fish appear to outgrow any tank you give them, delicate fish will remain small and sickly forever, and others in between (like goldfish) will get to a certain size and then stop growing.
With smaller tanks, water quality tends to drop faster and fluctuate more, especially in recirculated systems. If you had a hose going in one end and a drain on the other with new water constantly flowing through the tank, water quality would no longer be a problem and providing you fed the goldfish well, his back would be protruding from the water within a year.
I had a couple 1" goldfish in a tank last year over the winter. After one summer (3 months here) they were 6" long each. Watch out since goldfish are reasonbly hardy and will grow a lot in a pond… don’t expect to put them back into much less than a 30 gallon tank next winter.
Wow! Bonsai Fish!
I don’t know about you, but given a gym and a pile of junk food, I can regulate my growth pretty well.