“A different kettle of fish” is an idiom you might meet in British English. It means “a completely different thing”. I might say, “Learning French isn’t too bad, but learning Romanian is a different kettle of fish.” You might think this is a rather strange idiom. Kettles are things you boil water in to make tea, right? Well, yes, but there are also fish kettles which are oval-shaped saucepans that you can cook a whole fish in, particularly one that you might just have caught.
An old fish kettle
There is a whole range of fish kettles for sale on eBay.
In America, as far as I know, they call fish kettles either poachers or steamers, so you’d be unlikely to come across this idiom there. In the US the equivalent expression is “a whole new ball game“.
I thought I’d mention “a different kettle of fish” because it came up in conversation over the weekend. A similar fishy idiom exists in Romanian, although the exact wording has slipped the mind.