A fish species is considered endemic to a location if that is the only place it occurs naturally.  It may have evolved in that region or over time has become so adapted to that specific environment that it cannot survive elsewhere.  For example, Devil’s Hole Pupfish (Cyprinodon diabolis) is only found in Devil’s Hole, Nevada. 

Devil’s Hole Pupfish is endemic to Devil’s Hole, Nevada.

If a species is naturally located in multiple locations, it is considered a native species, but is not an endemic one.  Many endemic species are T&E species, threatened with extinction due to loss or modification of their habitat by agriculture, urbanization, or other human land uses. (fishionary.fisheries.org/endemic-species)