1, the sea water has high density and strong pressure, and marine fish will die when entering fresh water because the blood pressure of marine fish adapts to the sea water pressure, and the fresh water pressure is low after entering fresh water, and the blood pressure of marine fish exceeds the water pressure, resulting in the sudden rupture and death of blood vessels.

Fish contains salt necessary for the operation of the body. In order to prevent fresh water from diluting the necessary salt in the body, or to prevent fish from being salted into "salted fish" by seawater, fish has the ability to maintain the balance of salt concentration in body fluids. Freshwater fish excrete excess fresh water through the kidney to keep the salt concentration balance in the body; However, in order to avoid dehydration caused by the withdrawal of body fluids from seawater, marine fish must drink a lot of seawater to replenish water, and excessive salt in seawater is excreted through the kidneys to form salty urine with high concentration.

Since different fish have different excretory and metabolic mechanisms, freshwater fish should not be kept in seawater at will, or marine fish should not be put into fresh water, and there should be no substances in the water that may destroy the osmotic mechanism of fish, such as improper metal salts and other substances that can affect the pH of water.

In addition, there are some marine fish with strong adaptability, which can live in normal seawater and enter the environment where the estuary is almost fresh water, such as puffer fish. Therefore, it is not strict to explain with osmotic pressure.

2. I think the problem of fish's firmness is mainly caused by the environment and living habits. For example, some bottom-sea fish are soft and tender because of less activity, while wild fish in fresh water are firm if they live in running water with a lot of exercise. Guangdong's mandarin fish (grass carp) is an example. Those who live in fish ponds are rich in feed and generally have tender meat, while those who live in man-made rapids and eat high-protein feed are very strong and elastic. This kind of Guangdong is called "crispy pork carp".