Our cichlids made me bite shy years ago. No blood but ouchy hickeys even without fry or food present. Kind of the same as giving blood, the visual anticipation makes it worse. I haven't even had my arm hair pulled in years
Lupin. blunt knife cuts hurt more than sharp IME so that didn't make me braver.
Think of it like a sharp little mouth, built for shearing rather than tearing off a bite. Since even the largest loach can't really get any tissue of yours inside his mouth, he can't shear off a bite of it. You might get a little scuff, but it's not like an arrowana taking your thumb off.