There are many variations of this popular idea, but some the main formulations would be:

  1. Fish will never "outgrow" their fishtank (small fishbowls will result in smaller fishes than larger ones, for same species and diet).

  2. The larger the fishtank, the larger a fish will grow (essentially the same idea, from a different angle).

Is there any truth to such claims?

(despite having a hard time finding online traces of these claims, I hope they are pervasive enough in everyday popular culture to deserve mention here)

  • If it helps, I have also heard this claim. Jul 8, 2011 at 4:54
  • I have heard this. When you look at a gold fish in tank it is maybe 5-8cm long, but in the wild or large pond I have seen them 30cm.
    – Craig
    Jul 8, 2011 at 5:25
  • I did an answer to this question and then realized that you were probably wondering about fish in general. The link you gave talked about goldfish so at first I had assumed that that's what you meant. Sorry if that wasn't the case. Jul 8, 2011 at 5:46
  • @nalgenegirl: yes, I was curious about fishes in general (no plan to really get a laser-equipped goldfish/shark/tuna myself)... But I would settle for a goldfish-specific answer. My main issue, though, is with the claim that environment essentially defines the size of the (gold)fish (over a single generation)... Unfortunately, none of the links I've seen so far really address this particular aspect. :-/
    – Dave
    Jul 10, 2011 at 15:33
  • The plural of fish is fish, not fishes.
    – psusi
    Jul 11, 2011 at 14:00

2 Answers 2


The Goldfish and Aquarium Board (linked from the Goldfish Society of America) notes that:

He may look like a tiny thing but goldfish live long lives and grow continuously.

This BBC article tells of a woman who had a 2lb goldfish.

(From the same article):

But Dick Mills, secretary of the Federation of British Aquatic Societies (FBAS), said Goldie's size was "not that unusual".

"I would think there are probably a few bigger goldfish that people don't think of as record holders, perhaps in ornamental lakes.

"Goldfish are very long lived animals and consequently if they're in the right environment they will get large."

The Federation of British Aquatic Societies single-tail goldfish care sheet notes that they can grow up to 350mm (almost 14 inches) but rarely do as most are kept in aquariums. The Pond Care sheet also notes that "Pond Goldfish will grow to around 200mm/8" depending on the size of pond as they tend to adapt their size to the environment."

The goldfish websites did note, however, that in order to be happy, healthy fish they do need at least a 10 gallon tank. As the link you sourced noted, small goldfish bowls are unhealthy and do not allow proper space or gas exchange.

If you want a goldfish the size of a shark, though, I think you are out of luck.


"The larger the fishtank, the larger a fish will grow" is certainly incorrect in general. The ocean is essentially unbounded as far as most species goes and fish have a characteristic size range. I would think that many large lakes would also amount to "essentially unbounded" as far as the large majority of species goes.

  • 1
    Hello, while I understand you can't find a reference, it is still compulsory here. Please consider looking harder or making your answer a comment.
    – Sklivvz
    Aug 26, 2011 at 17:48
  • 2
    You'd be wrong about lakes and oceans being unbounded. They restrict growth through predation and access to food resources, two things that don't happen in an acquarium setting. Fish tend to grow as long as they have food, space, and aren't killed by outside factors (disease, predation, accident). That's why fish in a captive setting can grow to much larger sizes than the same species in the wild (at least for some species).
    – jwenting
    Aug 30, 2011 at 7:12
  • @jwenting "Fish tend to grow as long as they have..." is begging the question. There's nothing in, for instance, google.com/… that suggests that fish are immune to allometric scaling laws. Aug 30, 2011 at 21:17

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