another kettle of fish

be another/a different kettle of fish (altogether) (completely different from something or someone else that has been talked about) — совсем другое дело; и сравнивать нельзя

Example: Andy was never very interested in school, but Anna, now she was a completely different kettle of fish. (CIDI) — Энди никогда особо учебой не интересовался, а вот Анна — она совсем другое дело.

see also
[another cup of tea]