Dr. Tim's First Defense (Freshwater)DrTim's First Defense was developed to help fish adapt to new environments by assisting the fishes' natural immune system. Environmental Changes Cause Fish Stress Changing the environment of a fish causes stress that robs fish of essential nutrients and can weaken their immune system. First Defense is scientifically designed to provide stress relief and support the immune system, helping fish cope with stress due to bagging and shipping from the store