With some fishes the young and adults look very different. Here are some examples.

French Angelfish

Pomacanthus paru
french angelfish, young
Pomacanthus paru
french angelfish, intermediate
Pomacanthus paru
french angelfish, intermediate
Pomacanthus paru
french angelfish, intermediate
Pomacanthus paru
french angelfish

Queen Angelfish

Holacanthus ciliaris
queen angelfish, young
Holacanthus ciliaris
queen angelfish

Spotted drum

Equetus punctatus
spotted drum, young
spotted drum-young
spotted drum-ini
Equetus punctatus
spotted drum, intermediate
spotted drum-ini
Equetus punctatus
spotted drum
Equetus punctatus
spotted drum
Myripristis jacobus, Equetus punctatus
blackbar soldierfish and spotted drum

Damselfishes

Pomacanthus paru, Microspathodon chrysurus
yellowtail damselfish and french angelfish, young
Microspathodon chrysurus
yellowtail damselfish
Microspathodon chrysurus
yellowtail damselfish
Stegastes planifrons
threespot damselfish, young
Stegastes planifrons
threespot damselfish

Parrotfishes

Scarus iserti, Sparisoma viride
striped (initial) en stoplight (young) parrotfish
Sparisoma viride
stoplight parrotfish, initial
Sparisoma viride
stoplight parrotfish
Sparisoma aurofrenatum
redband parrotfish, initial
Sparisoma aurofrenatum
redband parrotfish
Scarus vetula
queen parrotfish, initial
Scarus vetula
queen parrotfish
Scarus taeniopterus
princess parrotfish, initial
Scarus taeniopterus
princess parrotfish

Wrasses

yellowhead wrasse, young
Halichoeres garnoti
yellowhead wrasse, initial
Halichoeres garnoti
yellowhead wrasse, initial
Halichoeres garnoti
yellowhead wrasse
bluehead, young and initial
Thalassaoma bifasciatum
bluehead, young and adult
spanish hogfish
spanish hogfish-juv