Young Blue Dempseys are prone to parasitic illness... As they mature their resistance increases... Chances are your younger/smaller ones got it and it made it's way to the larger/older ones...
I use UV lights to protect against this. They are expensive, but so is replacing Blue Dempseys...
I've known MacFish for a quite a while (hi Gary) and can vouch he knows his Blue Dempseys. The method he is suggesting is the common approach. I personally do not have experience with it as UV lights have kept me out of this pinch.