excedrin, since I know your tank care is consistent, I'm leaning towards these possibles:
A change in your water company treatment (it's good to have a chlorine test on hand, to see if chlorine/chloramines remain after normal dose);
Your new water conditioner may have a different efficiency than your old one. either, or both of these 2 issues in combination could've damaged the fish's gills and yet not be detectible at this point. ;
Maybe or maybe not--> you did more sand turning last time than usual- I think you posted about sand concerns?
and finally, I've read posts that EBJDs are known to be more delicate than regulars and to die unexpectedly. so in this case, they could be disproportionately dying off like "canaries in the coal mine" without your tank being extremely off. just somewhat 'off' in water parameters, and briefly.
I am not informed enough to know, but if you have different colors from the same genetic batch as the EB's maybe they could also be weaker than regulars. (?)
Sometimes things pan out badly. It doesn't always mean that we altered the conditions so much that we should have known ahead of time.
Hang in there.