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When I see fish in an aquarium, they never seem to stop swimming. This is in contrast to e.g. the penguins, who spend a lot more time just standing there.

Is this true in the wild or specific to aquariums for some reason? Is there a biological or evolutionary reason this is so? Is it a special case of some more general principle? (It occurs to me that fish are much smaller than penguins.)

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    $\begingroup$ I'm guessing it has to do with staying in one place - currents constantly move and change, so fish need to adjust with them - while penguins, for example, have their weight and the ability to lean if the wind blows. $\endgroup$ – Zxyrra Dec 13 '16 at 3:31
  • $\begingroup$ Most sharks have to keep swimming in order to keep the gills supplied with oxygenated water. However, some can stay stationary. You can look up for a more convincing reference. $\endgroup$ – WYSIWYG Dec 13 '16 at 11:48
  • $\begingroup$ Wonderful question. $\endgroup$ – Always Confused Dec 13 '16 at 15:38
  • $\begingroup$ If one of the answers below have answered your query, consider accepting it by clicking the green checkmark, giving you and the other user some reputation points. This aids in closing the question as solved. There is no obligation to do it either. $\endgroup$ – Imtiaz Raqib Apr 26 '17 at 2:58
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A research, detailed in a 2012 issue of the journal PLoS Biology, suggests the fish do indeed take naps and can experience sleepless nights.

Another research with Pacific sand lance species, found them utilize a one of a kind methodology of tunneling into sand to rest and moderate vitality, and to maintain a strategic distance from predation.

Therefore, we can assume that fish sleep cannot be related to a terrestrial. As @Zxyrra has mentioned, with the currents being random in their direction, we can connect the reasoning of the continuous movement of the fishes as an effort to stabilize themselves.

On the aspect of any evolution or biological reasoning, fish sleep is still an ongoing research in marine biology. An interesting research have opened up a case in which through evolution, cavefishes have lost sleep. Information from this research could help you understand the evolutionary relationship of fish and sleep, in atleast, a broad scale.

UPDATE: Unique method of sleep in dolphins.

At the point when it's an ideal opportunity to rest, a dolphin will close down just a single side of the equator of its mind, and close the inverse eye (the left eye will be shut when the correct portion of the cerebrum dozes, and the other way around). Amid this time, the other portion of the cerebrum screens what's going in nature and controls breathing capacities. This process is termed as unihemispheric slow-wave sleep.

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  1. Many fish do not sleep as we know it, most do however "sleep" while awake or sleep for very brief stretches of time. That alone means you are more likely to see them active than say a dog or rat.

  2. Not all fish are in motion. Ambush predators can wait motionless for hours, but active hunter and foragers (aka most fish) will be in motion as long as they are not resting, becasue they are looking for food. Just like rats, mice, or other foragers.

  3. Fish will drift if not in motion so most hide when resting or sleeping which makes them harder to notice. That means you will only see active fish. This simple observation bias is responsible for most of it.

I had to change this because recent studies have changed what we know about fish. This is why I love Stack Exchange, it makes me reseach things I never would otherwise. http://www.nature.com/news/2007/071016/full/news.2007.167.html

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    $\begingroup$ Do you have a citation on fish not sleeping? Pretty much every organism with a brain that has been tested sleeps, from fruit flies to whales. $\endgroup$ – Bryan Krause Dec 13 '16 at 4:52
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    $\begingroup$ @BryanKrause Apparently, some fish (or perhaps even cetaceans) keep one hemisphere active to keep swimming and put the other to rest. Then they switch them after a while. I remember reading this somewhere. I have to look up for a reference. $\endgroup$ – WYSIWYG Dec 13 '16 at 6:09
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    $\begingroup$ @WYSIWYG - yep, this is well known, and it is also known that other animals can also exhibit some sort of "local sleep" that doesn't cover the whole brain. That's a lot different than "never sleeping" $\endgroup$ – Bryan Krause Dec 13 '16 at 15:43
  • $\begingroup$ So you'd put most of it down to the observation bias, and the difference between fish + penguins is that penguins can get away with resting in the open because they won't get pushed around by the current? $\endgroup$ – Eli Rose Jan 11 '17 at 2:22

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