Scientists inspected some fishes living in seawater and found that they are different in body shape, size, type, etc., but their gills have a special structure that can eliminate salt. Scientists call it chloride secreting cell tissue. The scientists also inspected some fishes living in freshwater and found that although freshwater fish are also different in body shape, size, type, etc., there is no such chloride secreting cell tissue on the gills. 
the scientists believe that this kind of tissue is the reason why marine fish live in the sea for a long time, but it does not have a salty taste.
There is a special structure on the gills of sea fish that can remove salt.