In this article we present the process followed to design a case study entitled Quién mató a Nemo Blue? (Who Killed Nemo Blue?). The case is classified as Design for a solution and is addressed to Chemistry students aged 14 - 16. The case is developed through eight different stages that show different aspects of the problem to study.The case is aimed to improve knowledge about water solutions and environmental attitudes regarding groundwater pollution, contained in the curriculum of Secondary Compulsory Education. At the same time, we intend to develop some of the key competences proposed by the new Spanish Education Law (LOE).