Finned Fish
Expert Removes candidates Used in: Sudoku, Killer Sudoku
An almost-perfect fish with one or two extra candidates (the “fin”) in a single box. The eliminations still hold, but only for cover cells that also share that box with the fin.
How to spot it
An almost-fish spoiled by one or two extra candidates (the fin) sitting in a single box.
What you can conclude
Keep only the eliminations whose cells also share that box with the fin.
Worked example
An X-Wing on 6 with one stray 6 in a box still removes 6, but only from cover cells that share that box.