Q: Our small pond in front of our house has been very low. It developed red algae a month ago. My husband dipped his finger in the water and there was green algae underneath the red algae. Can we kill the algae? Is it harmful to the fish?
A: Algae in a pond is most often caused by high nutrient concentrations. Algae is a plant so it responds to fertilizer just like your flowers do. First, make sure absolutely no fertilizer from the lawn runs into the pond. Second, use a lawn rake to remove as much algae from the water surface as you possibly can. While algae does not hurt fish per se, if lots of it dies at the same time it will remove oxygen from the water and the fish will suffocate. There are herbicides registered to use for algae control but you must be very careful to kill only small areas of algae (or weeds) at a time. I have a guide to managing ponds and controlling weeds at http://www.walterreeves.com/gardening-q-and-a/fish-ponds-managing/.