Lepidophagy

Lepidophagy is a specialised feeding behaviour in fish that involves eating of scales of other fish. Lepidophagy is widespread, having been independently evolved in at least five freshwater families and seven marine families.

Lepidophagy has been reported in a range of fish including: Chanda nama (family Ambassidae),Plagiotremus (family Blenniidae),Terapon jarbua (family Terapontidae), a few Ariopsis and Neoarius species (family Ariidae), several pencil catfish (family Trichomycteridae), some piranha, Exodon paradoxus, Probolodus, Roeboides and Roeboexodon species (order Characiformes), along with both Perissodus species, all four Plecodus species, Xenochromis, Haplochromis welcommei, Docimodus, Corematodus and Genyochromis mento (family Cichlidae from the African Great Lakes).

...
Wikipedia