Then the factor is light, not the nutrients. Even if you use hormones, without sufficient nitrate and/or phosphorus, the algae will not be so good. However, generally there are many species of algae that can grow even under lower lighting conditions, which may or may not be the type of algae you want for the ranchus depending on exactly how much light is lacking. Can try putting a simple light setup over it to compensate for a few extra hours of light.