The rate of oxygen diffusion in freshwater fish habitats depends on environmental factors, including temperature and dissolved oxygen concentration. This Demonstration uses Fick’s law for the effects of temperature on the oxygen concentration and rate of diffusion in water. Dissolved oxygen concentration less than 2 mg/L (red region) creates conditions that are unlivable for fish. Diffusion rates of oxygen in the range 2–4 mg/L (yellow region) create conditions that are livable with struggling. When the diffusion rate is greater than 4 mg/L (green region), conditions are comfortably livable.