This mixed methods study described preservice teachers' conceptions of tides and explored the efficacy of integrating online data into inquiry-based instruction. Data sources included a multiple-choice assessment and in-depth interviews. A total of 79 participants in secondary, middle, and early childhood teacher education programs completed the multiple-choice assessment of their baseline knowledge of tides-related concepts. A sub-group of 29 participants also was interviewed to explore their understanding of tides in more detail before instruction. Eighteen of those 29 teachers participated in the instruction, were interviewed again after the instruction, and completed the multiple-choice assessment as a posttest. The interview data sets were analyzed via a constant comparative method in order to produce profiles of each participant's pre- and post-instruction conceptual understandings of tides. Additional quantitative analysis consisted of a paired-sample t-test, which investigated the changes in scores before and after the instructional intervention. Before instruction, all participants held alternative or alternative fragments as their conceptual understandings of tides. After completing the inquiry-based instruction that integrated online tidal data, participants were more likely to hold a scientific conceptual understanding. After instruction, some preservice teachers continued to hold on to the conception that the rotation of the moon around the Earth during one 24-hour period causes the tides to move with the moon. The quantitative results, however, indicated that pre- to post-instruction gains were significant. The findings of this study provide evidence that integrating Web-based archived data into inquiry-based instruction can be used to effectively promote conceptual change among preservice teachers.