Algae are single celled plants. They occur in primarily two forms, one form composes strands which can get so long they appear as "hair" or string and is referred to as "String Algae".

Fortunately, String Algae is very, very easy to control.

The second form of algae is a bloom of the single celled free floating type, such as the tiny plant above as seen under the microscope. When you get an algae bloom in your pond, the fish disappear in the green soup.
Algae, as plants go, hardly differs from reegular plants on your desk. During the day when the sun shines on them, they MAKE oxygen and consume carbon dioxide. Cool, for the water and the fish.

But at night it's different. At night the algae CONSUME oxygen and produce carbon dioxide. That's bad for the fish.

But; most of the time, algae consumes NITRATES, which is good. They use nitrates to grow and live. So in order to keep nitrates down, or, if your nitrates get high, greenwater or other algaes are the solution and the result.

Here's why algae matters. It differs from one form of algae from another. String algae is one kind of problem and green water is another.