Dude I've had that issue before, where it feels like a phantom disease or something is wiping out your fish, no sign of illness, or aggression towards the lost ones, just a new body every week.
I started a special quarentine thread just to help prevent these kind of phantom losses but haven't got much in the way of participation yet :
I mean I either have planted tanks or to large to treat tanks and it is next to impossible to catch the fish sometimes without deconstructing a tank so much that it is damaging so I was looking for a more thurough method of quarentining.
Sorry man beautiful fish, Loosing one like that really ruins your week.