It is *possible* that the pool guy really meant "high phosphates". We test for that on a semi-regular basis, not nitrates. Phosphates may be considered to be a food for algae, but does not *cause* algae. As long as you have free chlorine (FE), there should be no algae to use the phosphates as a food source. As the other folks recommended, please read the pool school info. You just need to follow the SLAM process and you should be in good shape.