One day i mysteriously noticed one of my bichers belly up, no signs of struggle days or even hours before everything looked fine, i removed the carcuss and doubl checked all death possibilities, the next day 3 belly up bichers and my tsnxrtc hybrid lifleless, water temp fine, amonia nitrate fine, not signs of sickness, i pull my heater out of the water and the glass casing slides off. my guess is they were getting low impulse electric shock, very sad day.
Sorry for your loss.