Shanghai is restoring wetland to improve the ecosystem at Qingcaosha Reservoir, a major source of the city\'s tap water since the last two years. Experiments are being carried out at the reservoir, at the mouth of the Yangtze River delta, to reduce the threat of algae by introducing algae-eating fish. Signs of eutrophication, which leads to excessive plant growth and decay, had been spotted at the country\'s largest reservoir, which has been the cleanest source of tap water for more than the 13 million Shanghai residents. The reservoir is also facing threats of salt tides, with five alerts last year. \"We face constant pressure mainly triggered by pollution upstream,\" said Shen Yichen, general manager of the Chengtou Raw Water Co, the reservoir\'s operator. \"The water safety is in danger if the polluted water stays in the reservoir for too long and algae begins to grow.\" The reservoir, with its Level-II water quality, the second highest in a five-tier quality standard, provides 80 percent of water to the downtown area. More than 20,000 sets of data concerning reservoir status are collected each day for monitoring, officials said. That will increase to 40,000 sets by 2015. Restoring the wetland will help reduce the relatively high nitrogen and phosphorus content in the water, said Shen. Algae-eating fish are being introduced to the reservoir this year to curb the algae growth and improve the ecosystem. Shen said it might take years to see the effect of the measures but the moves were in the \"right direction.\" City water authorities said the compounds and pollutants at the mouth of Yangtze River were mainly from upstream factories, and the reservoir was also threatened by emergencies such as oil spills and ship collisions.