When they are under sea, river, lake and pond etc, We call them '물고기'. We also call them in a fish basin '물고기'. But They are food, We call them '생선'. A fish seller says "신선한 생선이 있습니다", There are foods made of fish like 생선회, 생선까스.

고기 means a meat, 물고기 is a compound word 물(water)+고기(meat). it shows a meat that comes from water(lake, river, or sea). Accordingly, that is a fish. 생선(生鮮) means a fresh fish as it is, that do not salt, or smoke with to preserve it.