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Study of Water Purification in Greenhouse Pond for Overwintering Tilapia Using Duckweed

Floating duckweed( Lemna minor Linn) frames were set up in greenhouse ponds where Nile tilapia( Oreochromis niloticus) was cultured. The floating duckweed frames were divided into six groups containing various amounts of duckweeds. The growth speed of duckweed and the water parameters,including dissolve oxygen,p H and nutritious salts( total phosphorus,total nitrogen,ammonia nitrogen,nitrite nitrogen) were monitored during the whole courseof the experiments. The results showed that the growth speeds of duckweed were dependent on the densities of the duckweed. Duckweeds with appropriate densities were able to enhance the dissolve oxygen,stabilize the p H,absorb the nutritious salts in the cultured ponds,so as to purify the cultured water and keep water quality. The density of the duckweed with 660 ~ 675 g /m2 was optimal for keeping the water fresh. The results will shed a new light on the greenhouse pond water management using floating duckweed frame.