All fish is different, but you can divide it into two major categories - red fatty fish and white fish. Red fatty fish would be things like salmon - it is very healthy because in addition to having a clean nutrition profile (lots of protein, no carbs), it has healthy fats with tons of omegas. White fish is cleaner in terms of fat, so it is a bit less healthy because there are less fatty acids, but this kind of fish may fit better those who want to avoid fat and are on a very strict diet. An example of such fish is tuna.