The main difference between freshwater fish and saltwater fish is that freshwater fish need to rid themselves of excess water and saltwater fish need to rid themselves of excess salt. Since there is not a lot of salt in the water of freshwater fish, they must do what they can to keep salt in their bodies. It is opposite for saltwater fish.
Their physiology is very different as well. Freshwater fish have kidneys that allow more water to pass through them, they have gills that keep salt in the body, and they have scales that allow only a certain amount of water to pass through.
When you draw the diagram you should show a picture of a freshwater fish and a picture of a saltwater fish. Show and describe what their physiological differences are. Also describe the flow of water throughout their bodies and how their body parts diffuse water and salt differently.