Nitrate is the final product of bacteria’s decomposition of protein. It is also the product of nitrifying bacteria’s oxidation of NH₃ ,NO₂ .the resource of NO₃ is fish’s excreta, residual food , and dead fish . NO₃ is also the nutrient resource lf algae .when NO₃   level is too high , it will result in overgrowth of algae. Meanwhile, it will influence fish’s health when living in high NO₂ surroundings. It also will restrain water plant’s absorption of Ca .Ma and Fe ,and cause