Algae is only harmful to plants when it starts getting bad, a little algae is not harmful. Algae grows where light & water are in the same place. Algae is a plant so most of the chemical stuff you use to kill it will kill your plants first. HOWEVER there are some new products on the market that work very well for stopping algae formation without killing your plants. The easiest way to stop algae is to block the light from shining on that spot. Poly film (also known as panda film) which is black on one side and white on the other is ideal for blocking the light from the soil/hydroponic growing medium.