How does pond scum eat?

Pond scum consists of algae. Algae is an autotroph, meaning they do not need to eat. Instead, they produce food internally from carbon dioxide, water, and sunlight using photosynthesis.

Pond scum is a algae, a plant. It needs sunlight. If you want to not have pond scum, you need to aerate the water. Avoid plant fertilizers as they will just make the pond scum grow faster.