
A different kettle of fish
Idiomatic Nuance
This phrase implies contrast, usually more than just a simple difference, and often suggests a situation is unique or unusual. 
Cultural Origin
This idiom is of British origin, so it might be more recognized and understood in UK English than in other varieties of English. 
Avoid Literality
The phrase doesn't relate to fish or kettles; understand it's a figurative expression for saying something is different. 