Fish need oxygen.
They just collect it differently from animals with lungs.
Water contains dissolved oxygen gas.
Fish move that water across gills, where oxygen can pass into the bloodstream.
Water contains oxygen
The oxygen fish use is not usually taken by splitting water molecules apart.
Instead, oxygen from the atmosphere and from photosynthetic organisms dissolves in water.
That dissolved gas is what the gills collect.
Gills provide a huge surface area
Fish gills contain many thin filaments.
Those filaments create a large surface area packed into a relatively small space.
They also contain many tiny blood vessels.
The thin surfaces make it easier for gases to move between water and blood.
Oxygen moves into the blood
As water flows over the gills, oxygen moves from the water into the bloodstream.
Carbon dioxide moves the opposite direction and leaves the body.
This exchange depends on differences in gas concentration across the gill surface.
Why are fish gills so efficient?
Many fish use a system called countercurrent exchange.
Water moves across the gill in one direction while blood flows in the opposite direction.
That arrangement helps maintain a useful oxygen difference across more of the gill surface.
The fish can extract oxygen efficiently even though water contains much less oxygen than air.
Why can fish suffocate in water?
Because water does not always contain enough dissolved oxygen.
Warm water generally holds less oxygen than cold water.
Pollution, algae blooms, overcrowding, and stagnant conditions can also reduce oxygen.
A fish can therefore be surrounded by water and still be unable to get enough oxygen.
Why do many fish die out of water?
Gills are designed to work while supported and separated by water.
Out of water, delicate gill structures can collapse or stick together.
That drastically reduces the surface available for gas exchange.
So fish are not splitting water molecules apart to get oxygen.
They are collecting oxygen gas that was already dissolved in the water.
Gills are basically underwater breathing equipment with a very different design.
A related animal question is Why do fish swim in schools?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: Why do fish swim in schools?, Why does the ocean look blue?, and Why is the ocean salty?