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Over half of the fish products that end up on our dining tables today are grown in fish farms (NOAA Fisheries home page). Unfortunately, fish farming is causing sea or ocean pollution, because of fish faeces, secretion and feed residues that are released into the water. In addition, there are problems caused by fish diseases, parasites, chemicals and medications, mainly antibiotics. (Mereorganismide kultiv. home page) Wastewater from fish farms can be cleaned with various filters, but these processes are relatively expensive. The best solution would be to use natural resources, for example algae, which get essential nutrients for their survival from the nutrient-dense wastewater and thus work as “water purifiers”. These algae could be additionally utilised for other purposes, like for developing new natural materials. This research paper is relevant as it contributes to the development of environmentally friendly fish farming and aims to help keep the Baltic Sea clean. As an enclosed sea, the Baltic Sea is especially vulnerable, so all measures to reduce pollution and maintain ecological balances are extremely important. Fish is an important part of the human diet. So fish farming helps to keep the ecological balance of global oceans by sparing edible fish living in the wild from being caught. There is only one fish farm in Estonia, where fish is grown in the sea, in Kesknõmme by Tagalaht bay in Saaremaa Island. However, the opportunities to farm fish in the sea would be much bigger when environmentally friendly technologies could be introduced. Currently, such technology is being developed in Kesknõmme, using algae cultivation to clean the wastewater. The objective of this research paper is to evaluate which algae species would be the most suitable for cultivating in tanks and cleaning wastewater from the fish farms. The research questions are following: • What would be the most suitable algae species to cultivate for cleaning wastewater from fish farms? • Which algae species can be the healthiest and survive the longest in an artificial living environment in tanks? The hypotheses made are the following: • There are algae species in the Baltic Sea that can be successfully cultivated in artificial conditions inside tanks. • The most suitable algae species for cleaning wastewater are thin and fast growing Cladophora glomerata and Ulva intestinalis. This study has been made in cooperation with the science- and development project “Treatment of marine water-based fish farm waters by cultivating macro algae” run by Estonian Marine Institute of the University of Tartu. The head of the project is the lead research professor Georg Martin from the University of Tartu, who is also one of the supervisors of this study. During the project, different algae species are cultivated in tanks. Their oxygen production is measured and, based on the outcomes, their well-being and ability to clean wastewater is assessed. This research paper consists of two parts: a theoretical and a practical part. The theoretical part gives a brief introduction about fish farming and about its negative impact on the environment. The algae species used in the Marine Institute project are presented more in depth: their physiology and their ability to clean water. In the practical part, the oxygen produced by the algae in the water tanks and by the algae grown naturally in the sea is measured and their biomass/dry mass is determined. From that the oxygen production is calculated (mgO2/(g∙h). Chemical samples taken from the tanks are also analysed. They show the change in the amount of chemical pollutants and the algae’s abilities to bind those substances. After that the conclusion is made and the most suitable algae species for cleaning wastewater from fish farms and for surviving the longest in artificial environment are assessed.