What causes foam in my pool and how do I fix it?

Low calcium hardness or high phosphates from sunscreen, sweat, and leaves are the main suspects. Too much algaecide can also cause it. Test your water first, then add calcium hardness increaser if low, use a phosphate remover if high, and shock the pool. A clarifier or filter rinse helps push out the crud. Don’t drain and refill unless it’s really bad.