Two common varieties of fish - mackeral (bangada) and sardine (tarle), are considered to be very good for health, as they contain fatty acids of the type that do not raise blood cholesterol.
Fish such as pom fret (paplet), king fish (iswan) etc and prawns contain fats of the type that are not at all good for health.
Even Shark, which contains lots of Vitamin A and D, does not contain 'good' fat.