A different kettle of fish
An alternative; a different thing altogether.
Before we can get to grips with 'a different kettle of fish' we need to know what 'a kettle of fish' is when it isn't different.
Of course a fish kettle isn't the kind of kettle you would use to make tea, it's just a fish saucepan - usually one large enough for a whole fish.
The expression 'a kettle of fish' means 'a mess'/'a muddle' and is often extended to 'a pretty/nice kettle of fish'.
'A different kettle of fish', and I have to say it, is a different kettle of fish. It means 'something different from the thing before'. For example, we might offer to have a friend stay for a few days but remark that a stay of a few months would be a different kettle of fish.
The expression dates from the late 19th century and was found most commonly in Scotland and the north of England (where fish kettles were and still are quite commonplace). This early citation comes from a report of a parliamentary debate on the Irish question, in the Carlisle Patriot newspaper, June 1889:
"To enable them to manage their own local affairs will not satisfy Irishmen. What they want is a very different kettle of fish."
See also, the meaning and origin of 'a kettle of fish'.