There are products, which are actually fungicides, which are marketed as being able to slow the return of mildew and algae. Some power washing companies promote them. However, we do not use them for one primary reason. The fine print will usually indicate that the product should not be used near any waterway, river, lake, pond, or reservoir, or similar wording to that effect. Although there are varying opinions, the eco-friendliness of such products is questionable and the primary reason we do not use them.