Using mini-aquaria experiments, a learning unit on the effects of light period on aquatic ecosystems was developed for 7th grade students. This guided inquiry unit was aimed at helping students understand basic ecological principles involved in relationships among physical, chemical, and biological components in aquatic ecosystems. It involved asking guiding questions to direct students' investigations and experimental designs, providing experimental data for analysis and interpretation, and facilitating students' construction of explanations. The students' achievements were assessed through a questionnaire, written documents, interviews, and classroom observation. In this study, students were encouraged to ask questions throughout the learning sequence, in which they did ask more intelligent questions, generate fruitful ideas, and finally develop their own understanding. Students gradually accumulated conceptual understanding through the designed learning activities, and both students and teachers had a positive attitude toward the learning unit. The minimum and maximum values of the students' perceptions on the learning unit were found to be 4.14 and 4.65 (on a 5-point scale). The mean and standard deviation of the 15 items on perception were 4.41 and 0.41, indicating a moderate level of approval. One advantage of this learning unit is in its low cost for implementation while producing effective learning activities, especially in schools with budgetary and time constraints. This study should inspire teachers to adapt and adopt the proposed activities to enhance students' understanding of the ecosystems. (Contains 4 tables and 1 figure.)