The idea that "grown medium fish can kill a male person but not a female person" seems to be a misconception or a misunderstanding. In general, fish do not have inherent properties that would cause them to harm or kill male individuals differently than female individuals.
If the discussion pertains to specific contexts, such as venomous fish or fish related to certain toxins, it is essential to note that reactions can vary based on individual health, size, allergies, and other factors, rather than gender alone.
For instance, certain species of fish can carry toxins, and the severity of an allergic reaction or poisoning can depend on an individual's immune response, which is not strictly determined by gender.
If you have a specific species or context in mind (like a myth, particular incident, or scientific study), please provide more details for a more accurate explanation.