The inquiry-based learning (IBL) approach is at the core of contemporary science education programs and aims to enable students to conduct research. In this study, an IBL activity was developed based on scientific concepts related to the respiratory systems of fish and scientific investigation skills. The developed activity was implemented for 2 hours in a 7th grade classroom of a public school. Data was collected through classroom observation, student worksheets, and semi-structured interview with the teacher following the lesson. Data analysis revealed that the activity was successfully implemented. The activity is a good resource for teachers who want to engage their students in IBL activities, seek materials that support students' scientific investigation skills, and want to support students' conceptual learning of scientific concepts. The general structure of this activity and how it involves inquiry-based learning provides an exemplary practice for the practitioners and researchers in the field.