Like most plants, they contain chlorophyll and are at the bottom of the food chain. They are helpful to fish because they dissolve oxygen into the water. They are also dangerous to fish because they might put too much oxygen into the water. The algae that are the smallest and the simplest are the most important. Green algae have cells with nuclei.
Stoneworts are large algae.
Euglenoids have greenish cells and swim with a flagellum. Dinoflagellates have one or more flagella. Diatoms are in two shells and make the water brown. Finally, if you ever see pond scum, it is probably algae.