I use a long necked bottle to harvest the eels; fill the bottle whit vinegar and eels up to the neck, put a dot of clean filter wool on top, and carefully fill the neck whit RO-water. a half hour later most of the eels are in the water, of course some vinegar will leach in, butt no significant amounts…
my only theory is that somehow an toxic mould got in to the culture, and trough de eels in to the fish. no idea if that’s actually possible..