a different kettle of fish

Translations
совсем другое дело
Dealing with children is a different kettle of fish compared to adults.
Работа с детьми — это совсем другое дело по сравнению со взрослыми.
Definitions
A different kettle of fish refers to a situation or issue that is entirely different from the one previously mentioned, often implying that it requires a different approach or consideration.
Words
a

article or possessive pronoun
неопределённый артикль
different

adjective
другой
kettle

noun
котёл
of

preposition
из
fish

noun
рыба