Which sentence states a fact?
Of all the species of damselfish, the blue chromis is the most striking in terms of color. The vitamins in a blue chromis's diet can affect how brightly the fish's colors appear.
A fact is something that can be proved by research or observation.
Napoleon Bonaparte was shorter than King Louis XVI.
The statement above is a fact. The statement can be proved by researching the height of each man and comparing them.
An opinion is something that a person believes, thinks, or feels. An opinion cannot be proved.
Napoleon Bonaparte was a better leader than King Louis XVI.
The statement above is an opinion. People can have different ideas about what makes someone a "better" leader, so the statement cannot be proved.
The second sentence states a fact.
The vitamins in a blue chromis's diet can affect how brightly the fish's colors appear.
It can be proved by reading a book about tropical fish.
The first sentence states an opinion.
Of all the species of damselfish, the blue chromis is the most striking in terms of color.
Most striking shows what a person believes, thinks, or feels. Another person might have a different opinion about which fish's color is most visually impressive.