Oh no, I'm so sorry to hear this
Is there any possibility of picked up a cheap second hand glass tank and a cheap sponge filter from somewhere, and trying moving them to that?
It's the only last ditch effort I can think of.
I'd wonder whether it was some kind of parasite or protazoan, if they hadn't rallied after the tank revamp before getting ill again. Perhaps some kind of contaminant that has tainted the tote and doesn't easily wash off?
@Colin_T , and other thoughts? I know that
@NCaquatics is very attached to these fish, and this is horrible