No. According to Australian Museum, many species of fishes lack scales. All the clingfishes (family Gobiesocidae) for example, are scaleless. Their bodies are protected by a thick layer of mucous.
There are a number of types of fish scales and they come in all sorts of shapes and sizes.
Four main kinds of scales and numerous variations of each kind:
Placoid (sharks and rays)
Cosmoid (lungfishes and some fossil fishes)
Ganoid (bichirs , Bowfin, paddlefishes, gars, sturgeons)
Cycloid and Ctenoid (most bony fishes)
The primary purpose of scales is to give the fish external protection. Larger, heavier scales supply more protection, but restrict movement, smaller, lighter scales offer less protection but allow for greater freedom of movement.
To learn more about fish scales, visit EarthLife.net.
Tip! Find Fish Care Guides at PetSmart.