Regardless of the species there are several characteristics that can be used to evaluate quality. The amount of natural firmness in fish flesh will vary from species to another. The gills are a good indicator of quality, and they should be bright red or pink with little visible mucous or slime. As a fish begins to age the gill color fades to light pink, then gray to dull brown and may become slimy in appearance. The skin of high quality fish has a bright shiny metallic appearance and scales adhere tightly to the skin.
Characteristic colors begin to fade as soon as a fish is removed from the water.
Many people rely on the eyes to judge quality but this indicator can be inconsistent. The eyes may be damaged during harvesting and handling even though overall quality may not be affected. If undamaged, look for bright clear eyes that protrude away from the head. As fish quality deteriorates the eyes will become cloudy and sunken. It's more difficult to evaluate the quality of fish fillets and steaks, but with practice consistent judgements can be made.