How fish swim
Anguilliform (eel) locomotion: body flexion waves from head to tail (S form)
Carangiform (caranx = horse mackerel) locomotion: tail (beyond the anus) moves (S/C form)
Thunniform (tuna): only the tailfin moves, the rest is stiff (C form)
Fish larvae, when just hatched, move in an S form. As they age, they advance towards swimming in a C form, though some species retain the S form.
For small fish, the water is relatively thick, like syrup, making it difficult to move through it. An S form allows these fish to ‘wriggle’ through the water. When in danger - needing to escape - a trick is useful: the fish bends its body (sharp C form) and ‘shoots’ out of the shape like a rubber band, getting a quick start.
Larger fish, C form swimmers, ‘beat’ their way through the water more powerfully. The water is relatively less syrup-like for larger fish.
Tuna use only their tailfin for swimming. The body of a tuna is very streamlined, an effect reinforced by its rigidity, and the tail is high and narrow. Tuna have many red muscle fibres (for endurance); they are true marathon types, suited to economical (efficient) and rapid swimming.
Pike swim by using their tail. The body is streamlined; its large tailfin moves in big, rapid strokes which can’t be sustained for long. Pike have white muscle fibres (for short, rapid performance); they are sprinter types, specialised to accelerate, suitable for lying in ambush.
Catfish have an elongated, streamlined body with a long strip of fin along the top and bottom of the body, instead of separate dorsal and ventral fins. The entire body moves in waves (S form) while swimming. Catfish are bottom fish that float in the water in search of food. Though they can endure long journeys - they have red muscle - the S form makes this inefficient.
Butterflyfish have a short, flat, high (disc-shaped) body, which gives them the ability to turn easily around its centre of gravity. As each fin is used separately, butterflyfish can easily dose small movements: they are specialised in manoeuvring.
In open, clear water the majority of fish are long-distance hunters, many with strong suction capability (for rapidly approaching food and then further reducing the distance by sucking it in). In complex environments (coral reefs, kelp beds, etc.), ambush hunters and selective eaters abound.