Phosphate
Phosphate is a nutrient in soil and water that feeds plants and algae.
Think of it this way
A pot of water sits on a counter with fertilizer pellets dissolving at the bottom. The dissolved minerals spread through the water, reaching plant roots. Those roots pull in the minerals and grow fast. Phosphate works the same way in soil and water — it dissolves and feeds whatever is rooted nearby.
Explaining phosphate by grade level
Ponds need some plant food in the water to stay healthy. But too much of one kind, called phosphate, makes green slime grow fast. The slime blocks light and uses up the air in the water. Fish and other pond life can get sick when that happens.