With some fishes the young and adults look very different. Here are some examples.
French Angelfish
french angelfish, young
french angelfish, intermediate
french angelfish, intermediate
french angelfish, intermediate
french angelfish
Queen Angelfish
queen angelfish, young
queen angelfish
Spotted drum
spotted drum, young
spotted drum-young
spotted drum-ini
spotted drum, intermediate
spotted drum-ini
spotted drum
spotted drum
blackbar soldierfish and spotted drum
Damselfishes
yellowtail damselfish and french angelfish, young
yellowtail damselfish
yellowtail damselfish
threespot damselfish, young
threespot damselfish
Parrotfishes
striped (initial) en stoplight (young) parrotfish
stoplight parrotfish, initial
stoplight parrotfish
redband parrotfish, initial
redband parrotfish
queen parrotfish, initial
queen parrotfish
princess parrotfish, initial
princess parrotfish
Wrasses
yellowhead wrasse, young
yellowhead wrasse, initial
yellowhead wrasse, initial
yellowhead wrasse
bluehead, young and initial
bluehead, young and adult
spanish hogfish
spanish hogfish-juv