Experimentation is a valuable way to build understanding in science. EcoXPT research looks at supporting authentic experiment-based inquiry within an immersive virtual ecosystem. EcoXPT includes a variety of investigative tools, including a Mesocosm tool that allows students to set up experimental pools to test interactions between variables. This paper looks at student use of the Mesocosm tool over time. Analysis of log file and video data demonstrates that students' use of this open-ended tool progresses towards a greater focus on gathering evidence and testing specific hypotheses.