Algae are microscopic plants — and the most common, troublesome aspect of pool maintenance. They can make your pool cloudy and murky. They also can clog your filter, and increase chlorine demand even before they are visible. Algae usually start in hard-to-reach crevices in the pool surface or areas of poor circulation. You need algae protection the most when water temperatures rise. That's why the middle of the swimming season is prime time for algae to develop. Prevention is by far the best way to fight algae.