mysterious death of black ghost knifefish

Hi all...
This morning, when I woke up, my 5-6 in black ghost knife fish was
dead. The day before, he had been eagerly swimming about, eating, etc.

My tank is a 55 gallon planted tank. There is driftwood that creates
plenty of hiding spaces for him. I only would see him when he wanted
to come out, but that was still pretty often as he warmed up to his
surroundings. I've had him probably 3 weeks.

The water quality is what I would consider very good: 0 ammonia and
nitrite, of course, but nitrates are also quite low. Yesterday (before
he died) they were between 5 and 10 ppm. The water is slightly acidic
(6.6-6.8) and quite soft--- about 3 degrees. I have 4 juvenile discus
in the tank that seem happy, which is why I keep the water so
pristine... (and, as I understand it, these condtions should be perfect
for the black ghost as well). So, any theories? I'm hoping to get
another of these great fish in the future, but I'd like to ponder what
went wrong with this one first.

Thanks very much for any suggestions,
Wes