Hi Joe,
This is pretty sad, I'm sorry to hear about this.
Somethings to think about...
You had a group of discus in tank that were fine...split the group...half to a planted tank..half stayed... The half that stayed have been having issues...the half in the planted tank are fine.
two things that comes to mind...
...something may be wrong with the water in the sick fishes tank... do you do a different water change regime between the bare grow out..and your planted tank...most do.... if so..I'd question my water supply...is this municipal water?
.... the med you dosed with..in your water parameters may have been toxic... I knew someone that could not use the medication ...fluke tabs in her water....she experienced multiple deaths on multiple occasions when she used it...I've never had an issue once....I can only attribute to water differences.
Sorry you lost them...on the off chance that the problem was due to something contagious...watch that planted tank closely.
hth,
al