What algae needs
Algae needs sunlight and nutrients. Remove either and it fades. In most ponds, nutrients are the easier thing to control.
Green water
Free-floating algae turns water green. It is common in spring and summer. A UV clarifier, more plants and better filtration clear it.
String algae
Hair-like string algae grows on rock and waterfalls. Remove it by hand, improve circulation and add plants that compete for nutrients. UV will not stop it.
Reduce nutrients
Feed fish lightly, keep leaves out with a net and skimmer and clean muck from the bottom. Fertilizer runoff from lawns adds nutrients, so direct runoff away from the pond.
Add shade and plants
Covering about half the surface with lilies and floating plants shades the water and uses up nutrients. Marginal plants and bog filters add more competition.
Be patient
A pond that took months to turn green may take weeks to clear. Make changes steadily and avoid quick fixes that upset the balance.
Cut the nutrients, add shade and plants and give it time.
