To solve a serious green algae problem, the present invention is to increase nitrogen and phosphorus, oversupplied nutrients, in a controllable state in order to make the nitrogen and the phosphorus in the water of a hose or a river in an appropriate state using green algae. To increase (culture) the green algae in a controllable state, the present invention makes air hoses (2) float on the surface of the water to generate buoyancy on both sides of a transparent hose (1), and floats the air hoses on the surface of the water or places the same on the ground. When supplying polluted water, green algae water for weed, and carbon dioxide through a water supply flange (3), a green algae water supply flange for weed (4) for increasing the green algae, and a carbon dioxide supply flange (5) through an entrance of the transparent hose, the water receives light while passing inside the transparent hose like intestines of an animal, and the temperature of the water inside the transparent hose is increased and becomes the temperature to let the green algae be proliferated. As the green algae for weed, mixed in the water due to photosynthesis, intakes carbon dioxide and produces oxygen and slowly flows toward the entrance, the green algae is proliferated with the nitrogen and the phosphorus, which are the nutrients in the water, as nutrients and reduces ingredients of the nitrogen and the phosphorus contained in the water. The present invention installs a green algae filter (7) which can separate the green algae in an outlet to discharge clean water. Since the hose is long, the carbon dioxide is in need as going toward a lower end. To this end, when connecting a carbon dioxide supply hose (5-1) having multiple holes inside the hose with the carbon dioxide supply flange (5), the present invention can equally supply the carbon dioxide inside the hose. The present invention can reduce the nitrogen and the phosphorus in the water as the green algae is proliferated while the water s
