This is literally just something I've heard--that a knife is not supposed to be used, nor put out, for the fish course (i.e. on Shabbos). I ask people wherever I go if they've heard of this. Some say no, some suggest maybe, and of course the first person who told me thought yes, so I realize there is a very good chance that this is a small-group minhag of some sort.
According to some, the suggested technique is to pick up a piece of gefilte fish with your fork, dip it into the chrein, put it on your plate and eat it with your fork (also cutting it with the side of your fork).
A very educated rabbi whom I asked suggested that the custom, if it exists, might have something to do with the mystical symbolism of fish as tzadikim.
Has anyone heard of this? Anyone know who does it, to what extent, and on what basis? Thanks!