Why? What’s the mechanism that tells an organism that it can only grow to a certain size, due to its environment, and no more?
Goldfish DO in fact outgrow their fishbowls. The idea that they don’t is a myth. What happens if you keep a goldfish in a small fishbowl for too long is that the buildup of hormones and other toxic secretions in the water first stunts the fish, then eventually kills it.
Goldfish kept in a pond or a very large aquarium can live for more than 20 years. People who keep them in small aquariums and fishbowls think that when the fish dies after a couple of years, it died of old age. No, it died because of grossly substandard environmental conditions.
Exactly. Goldfish don’t “outgrow” their fishbowls because they die of filth first.
Hah! I had to give away two goldfish just last week because they outgrew our 5-gallon tank.
I have a ‘rescued’ goldfish I brought home a month ago and dumped in our enormous fish tank (not much in there other than guppies and a humongous plecostomus). That goldfish is easily twice the size it was when I put it in - he is King of the Tank, which ain’t saying much, but there it is.
I was taught the rule of thumb >20 gallons of water per goldfish. The people I know who have follwoed that have goldfish 10-15 years old.
Goldfish are filthy, filthy things. They excrete like mad and it’s fortunate that they’re fairly tolerant of poor conditions such as small bowls with no filtration or stagnant ponds.
I’ve got a 3" long goldie in a 12-gallon tank with power filtration and on her own, she’ll put out enough waste to require montlhy filter changes and water changes. I can’t imagine how bad conditions would be in a half-gallon bowl.
Slightly off-topic: There was a news story a few years ago about a local family here in suburban northern Virginia that brought home (what they thought was) an adorable little pot-bellied pig as a pet. Said animal turned out not to be a pot-bellied pig but rather a piglet from an ordinary barnyard hog, and he grew to be something like 400 pounds. By then, he was a member of the family, and they were fighting with the county over whether they could continue to keep him in their home.
Apparently you have never read “A Fish Out of Water” 
When I learned this, I was told (or maybe I read it) that goldfish excrete growth-stunting chemicals into the water, which begin to act once they reach a certain proportion, and act more strongly the higher the concentration. Thus, the smaller the body of water holding the fish, the more these growth-stunters build up.
Never did know for sure if it ws hooey or not, but to answer the OP, that’s the “mechanism” that would provide this kind of feedback.
Didn’t anyone else read A Fish Out of Water as a child? I always assumed that the goldfish that I won at school events would end up outgrowing their bowls. Sadly, they always died before that could happen.
Hah! I see that CookingWithGas got there while I was looking it up.
You…you mean the death of my goldfish was…my fault??? :(:(
LOUNE wont have that problem. He has 2 big water turtles that will take care of any gold fish.
I don’t have goldfish, but couldn’t merely the switching out of water get rid of any build up of filth or growth-stunting hormones? If you did that every day would it affect the fish?
Sure that will help but plain tap water isn’t so great either.
Actually I’m surprised that more people aren’t aware of how unhealthy it is for a Goldfish or any fish for that matter to be stuck in such a closed environment without active filtration.
In my opinion it is unnecessarily cruel.
cue the betta debate
You asked for it.
My neighbor gave me a betta once when he moved crosscountry. I changed the water once a week but the poor fish was still in horrible condition in a tank he could not swim in for any distance. I couldn’t put him in the big fish tank since my tiger barbs would have ripped him to pieces. I couldn’t buy another tank. I couldn’t find anyone who’d take him and put him in a real fish tank. The poor thing lasted a year before fin rot got him.
I am all for eating meat. I’m all for sport hunting. I’m all for keeping animals in zoos. But that betta made me very very sad. I think bettas should be banned unless the store that sells them makes sure you know that they CANNOT live in those tiny 1/8th gallon tanks.
I got for Christmas one year a betta in one of those big glass vases with a peace lilly floating on top. The lilly soon died, so I threw it away, put some marbles in the bottom of the vase, and The Red Menace flourished for years! He lived for his little betta pellets twice a day. I kept a pail of tap water uncovered so there would be no chlorine in it, and every couple of days would scoop The Red Menace out, dump out the foul-smelling vase water, fill it up with water from the pail, and return the fish. All of this took less than three minutes, twice a week, and betta pellets lasted nearly a year. The Red Menace was a great pet! And very photogenic.
I wish someone had told me during my six-day experience as a goldfish owner.
We had a gold fish in a half gallon bowl for ten years.