Foxfish are a type of saltwater fish, mainly because their body structure and physiological functions have adapted to the seawater environment.

First of all, the gills of foxfish have a high-efficiency filtration function, which can efficiently filter out oxygen in seawater while eliminating excess salt. This adaptive trait allows the foxfish to live freely in high-salinity waters and meet its respiratory needs.

Secondly, the physiological functions of foxfish have also adapted to the temperature, pressure and other environmental factors of seawater. They are able to maintain stable life activities amidst fluctuations and changes in seawater, which is also an important feature of them as seawater fish.

To sum up, the reason why foxfish are saltwater fish is because their body structure and physiological functions are highly adapted to the seawater environment, allowing them to survive and reproduce in seawater.