For short-term stress, the most common reaction is one everybody recognizes — to flee from danger. In the wild, the cause is frequently a predator. In captivity, it can be a net that causes the reaction, as the keeper tries to capture a fish for closer examination or transfer to another tank.,When a fish senses this kind of danger, it triggers a short-term alarm reaction by releasing hormones, including adrenalin for its locomotory muscles. This will give it a shot of energy to escape quickly. The fish also releases cortisol. Problems occur because the fish’s body exchanges long-term health for a short-term boost to relieve the cause of stress — the adrenalin disturbs the fish’s natural osmoregulation (the balance of salt and water in its body) and the cortisol affects white blood cells and reduces the effectiveness of the immune system.,How Stress Can Affect Your Fishes’ Health Stress is a critical factor in fish health. It is so important, in fact, that scientists have studied it in detail, both in the wild and in captive fish. Stress is a very complicated subject that permeates every aspect of fish-keeping.