seems random, as I had 9 in the tank, one died after I put in some blanched spinach but none of them ate it but maybe the one that died did eat it since it was dead next to it. The other I found dead while holding food, so maybe it over ate? Should I be concerned, the two death occurred within a week of each other. The remaining 7 seems fine and active.
Did you wash it then boil it soft?