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Yes, you read that title correctly. I saw the claim in "Colonies in Space", hosted on the NSS site.

In a weightless space farm, it may be possible to raise fish without water. On Earth, when a fish is taken from water, gravity makes its gills collapse so that it cannot get oxygen. In weightless space these same fish might easily "swim" through an atmosphere of 100 percent humidity, keeping comfortably moist: hydroponic fish, if you will.

This sounds completely bonkers. Could astronauts aboard the ISS open up the fish tank and let them "swim" (or flail randomly) about the station?

Are there not problems with Oxygen availability? Even if the limit in air on Earth is collapse of the gills due to gravity, can a fish really survive getting air instead of water? Are there any credible references, or a sound scientific case that this is possible? Why hasn't it been attempted in experiments? Do gills even work that way?

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    $\begingroup$ Super question! Related; What about sea mammals like dolphins and whales? Oh my brain is turning... I feel a new question coming on. $\endgroup$ – James Jenkins Aug 14 '13 at 15:14
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    $\begingroup$ I'm afraid this question needs migration to Biology.SE. There are not enough biology experts here. A related question at Bio.SE: biology.stackexchange.com/questions/8318/… $\endgroup$ – Deer Hunter Aug 14 '13 at 15:59
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    $\begingroup$ @DeerHunter That's a possibility, we're still feeling out the scope with this type of question. Perhaps I should bring it up in the Biology.SE meta. $\endgroup$ – AlanSE Aug 14 '13 at 16:09
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    $\begingroup$ The positive side is that the lead link to a migrated question will always remain here. $\endgroup$ – Deer Hunter Aug 14 '13 at 16:30
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    $\begingroup$ I think the question should stay here. $\endgroup$ – James Jenkins Aug 14 '13 at 16:52
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There have been several experiments with fish in space; as far as I can tell, all have used water rather than humid air as the habitat. While there is no definitive answer available as of 2013 (lack of empirical research), present research suggests fish cannot really live in space without water.

It appears difficult to keep fish alive and healthy in a water environment in space. This quote from the NASA article points to some of the challenges.

This facility includes an improved water circulation system that monitors water conditions, removing waste while ensuring proper pressure and oxygen flow rates. The system’s design upgrades are based on lessons learned from previous habitats that flew on space shuttle missions STS-47, STS-65, and STS-90.

If we imagine a humid air environment that accounts for all these known hurdles, it is easy to imagine that significant air movement would be required to keep a healthy environment. It is difficult to imagine how you would move the air sufficiently without negatively impacting the fish. Air is about 784 times less dense then water, to me that means it is going to be 784 times harder to swim in air than it is in water. Without some kind of modification is difficult to imagine how a fish would be able to generate sufficient mobility in air to overcome the required air movement to keep the habitat healthy.

References

Related Swimming in air. Humans can swim in water at about 5MPH (8KPH), this is faster then some fish can swim but floating in space people are essentially unable to swim in air.

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  • $\begingroup$ Air is less dense, but it also produces way less resistance, so it's probably not quite 784 times harder to swim through. $\endgroup$ – Rikki-Tikki-Tavi Jan 23 '17 at 11:58
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    $\begingroup$ @Rikki-Tikki-Tavi I have edited the answer, to what degree it is difficult is open for further research, but in practice it close enough to impossible not to mater for this answer. $\endgroup$ – James Jenkins Jan 23 '17 at 13:42
  • $\begingroup$ What about oxygen-xenon atmosphere? ;-) $\endgroup$ – SF. Jan 23 '17 at 23:57
  • $\begingroup$ @SF. according to this sister site question xenon is NOT inert in the human body. $\endgroup$ – James Jenkins Jan 24 '17 at 0:55
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Gills aren't purely for respiration. Fish constantly excrete ammonia and urea from their gills and without sufficient water to wash the waste away this would soon cause death, much like how too small of an aquarium allows these substances to quickly accumulate to toxic levels (which begs the question about how waste and hygiene would be managed in the first place). Maybe a particular species of fish with a unique respiratory system and strong immunity could thrive in a novel microgravity environment though?

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    $\begingroup$ Is there any particular reason to suppose that the water required for waste removal would scale any differently than the water required for oxygenation? If not, a fish that can get enough oxygen (by whatever means) would seem likely to be able to survive in general. $\endgroup$ – Nathan Tuggy Jan 20 '17 at 0:19
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    $\begingroup$ @NathanTuggy The reason is that urea is much less soluble in air than in water. $\endgroup$ – Rikki-Tikki-Tavi Jan 23 '17 at 11:59
  • $\begingroup$ @Rikki-Tikki-Tavi: That should be in the answer (which is why I commented). $\endgroup$ – Nathan Tuggy Jan 23 '17 at 18:35

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