Q: Can one buy fish, such as salmon or tilapia from Cosco or a fish market?

A: Horav Shlomo Miller Shlit”a is of the opinion, that if the fish has its skin attached and you can see that it is of the kosher kind (it has scales), if the market service people are careful in their cleanliness and wash their knifes and cutting surfaces between costumers, it is permitted to buy that fish.
 
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a