system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do fish control where they are in the water? Why don’t they just float or sink?
gpt
Fish have a special organ called a "swim bladder" which regulates their buoyancy such that they tend to be about neutral, neither floating or sinking. Various sicknesses can cause problems with this balance which is why dying fish will tend to float up to the top of the tank when still alive.