Adding insulin to the water causes fish to exhibit slow behavior because insulin regulates blood sugar levels in the body. When fish are exposed to insulin in the water, it affects their metabolism and slows down their bodily functions. Insulin promotes the uptake of glucose, which is essential for providing energy to the cells. However, an excess of insulin can lead to hypoglycemia, a condition where blood sugar levels drop too low. This can result in decreased activity and lethargy in fish. Additionally, insulin also plays a crucial role in regulating growth and development. Excessive levels of insulin can inhibit growth and affect the overall health of the fish. Therefore, adding insulin to the water can have a significant impact on the behavior and well-being of fish, causing them to exhibit slow behavior.