What is the cause of Green Algae in my pond?
The green water is just a single-celled type of free-floating algae. The reason you didn’t have it before is that the duckweed grew over probably most of the surface of your pond and shaded the water below – without enough light, the algae can’t perform photosynthesis. And the roots of the duckweed (yes, they are there, just small) take up some of the nutriets out of the water, so they competed with the algae for food. This year, the algae gets plenty of light, plus the waste from the koi and their food are fertilizing your algae. You pump won’t have any effect on it other than maybe stirring up more nutrients from the bottom of the pond as it disturbs your water. There are a couple of things you can do to control the algae. The first is to restrict light to the pond. Get some more duckweed (and hope the koi don’t eat it) or some water lettuce, water hyacinth, or water lilies to cover the water. You could also rig up a cloth or screen shade, or add a dye (you can buy this at pond store
