a different kettle of fish

/ɐ dˈɪfɹənt kˈɛtəl ɒv fˈɪʃ/

/ɐ dˈɪfɹənt kˈɛɾəl ʌv fˈɪʃ/

an issue or subject that is not in any way connected to what one was talking about

Examples

1. Jackson, I think, is a different kettle of fish, although the seeds of removal are with Jefferson.

2. The first decade of the 20th century was a different kettle of fish.

3. But Joe Biden is a different kettle of fish.

4. That's a different kettle of fish entirely.