The term 'lengthy fish' can be solved by the three-letter word 'EEL'. Eels are elongated fish that can range in size from a few inches to over 13 feet in length. They inhabit both freshwater and saltwater environments, and can be found in tropical and temperate climates around the world. Eels have a slimy skin and lack scales, and they are known for their ability to move quickly and gracefully through the water.