A different kettle of fish
One early use of the idiom is from a newspaper called the Carlisle Debate in 1989, where when describing a parliamentary debate on the Irish Question, it said: ‘To enable them to manage their own affairs will not satisfy Irishmen. What they want is a very different kettle of fish.’
Riding a bike in London is a different kettle of fish from riding a bike in Amsterdam.
I had studied English in my home country, but studying at Bloomsbury International is a totally different kettle of fish!
‘Do you like your classmates?’
‘Well I like most of them. We get on really well, but there’s one who is a completely different kettle of fish.’