Kettle of fish
The expression dates from the 18th century and has developed two meanings. Firstly, the original one, a fine kettle of fish means that an awkward or disagreeable state of affairs has arisen. Secondly, a different kettle of fish means a new or different state of affairs has arisen. Both these figurative expressions derive from a kettle or pot of fish in the literal sense. The cooking of fish outdoors was a favourite picnic pastime of Border gentry during the early 18th century. The OED specifically mentions the river Tweed in this connection. Thus the fish were freshwater, probably salmon or trout. No one knows for sure how the figurative sense evolved from the literal, unless of course when fish were overcooked, it resulted in an awkward or disagreeable situation.