In order to split water into oxygen and hydrogen, we would have to supply energy.

In very large stars, helium gets turned into oxygen, and so on.

With the help of light and water carbon dioxide particles could be transformed into oxygen.

That's the process they use to convert sunlight and water into oxygen.

At sufficiently high temperatures, water will break apart into hydrogen and oxygen.

Where would you get the energy to actually split the water into hydrogen and oxygen?

The fish does not decompose water into hydrogen and oxygen.

It takes energy to separate water into oxygen and hydrogens.

An example is the breakdown of water into hydrogen and oxygen.

They take Carbon dioxide, which we breathe out and turn it into oxygen for us.