Think critically about why you did each step of the experiment, for example, what is the significance of there being no soap froth in the water, i.e. indicates that the water is not hard, the pH and adding the Na2S indicates something about different aspects of water quality, also you could compare the difference in moles of O2 before and after the Winkler titration and leaving it rest aside for a few days, what does that indicate about the health of this specific water body. You will learn the most if you really think through what are the purpose of those steps and the significance of those results based on what you learnt, formulate a sample response and then allowing us to review it-you will learn a lot more than if someone tells you the answer straight away.