Sashimi Swampfish: col C, col F (fins C8 C9) and row 3, row 7 on 3 (A7-3)
All the possible positions of the value 3 in two base cols are in the same two cover rows; except col C which have extra fins C8-3, C9-3, but each elimination sees all of the fins.
Hence if any of the fins were 3 then A7-3 still could not be 3, and 3 must either be in the fish or in one of the fins, so either way we can remove 3 from covers except bases that see all of the fins.
Therefore we can remove A7-3.
Explanation
Finned fish increase the utility of the Fish concept by expanding the number of hints produced, at the cost of limited eliminations. A prima facea look at fins says it breaks the core Fish covenant: N bases have only N places for value, hence the value is "locked into" these bases, but adding the restriction that each elimination sees all of the fins props-up the whole deal, so it's logically valid, it's just a bit slow Redge.
Note that BasicFisherman is S__TLOADS faster than ComplexFisherman coz it's much simpler. A genius would work-out how to find finned fish efficiently. I'm just a stupid programmer. This is the best I can do: A bit s__t; and it only gets worse from here, in terms of complexity, if not time.