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The Baltic Sea suffers from severe eutrophication – a state with massive algal blooms and dead bottom zones. These symptoms are caused by excess phosphorus and nitrogen that have been released by human activities and function as nutrients for algal growth. Unfortunately, nutrient rich bottom sediments often continue to feed eutrophication, although input from land has decreased. This study investigated a new material – activated limestone – that was added to nutrient rich bottom sediment with the ambition to stop nutrient release and thereby reduce eutrophication. Our results show that the activated limestone was very efficient in stopping leakage of phosphorus from eutrophic sediments collected at a fish farm on Åland from which fish feces have contributed to the nutrient load and accumulation in its surrounding waters. The phosphate release from the sediment was completely stopped by addition of 600 g/m2 activated limestone. In addition, the risk of potential side effects, such as greenhouse gas emissions, was measured and found to be low after addition of the activated limestone. Overall, the use of activated limestone can contribute to increasing the sustainability of fish farming, and to diversify the tools needed to curb eutrophication globally.