At present, the resins for nitrate removal in the aquarium market are confusing, including strong base anion resin 20 1x7, D20 1, and macroporous weak base anion resin D30 1. In fact, although these products can remove some nitrates through functional groups, they are not the best choice because they are not selective in nitrate removal and their exchange capacity is not fully utilized. D890, which I am proud of, is a special resin for selectively removing nitrate. After saturation of adsorption and exchange, 8- 10% NaCl solution is used repeatedly, only nitrate and nitrite are selected, and anions such as chloride ion, sulfate ion and carbonate ion in water will not be exchanged.