happy you got what you need! especially for the live plants. I have mine chilling in tap water.
Okay, if you have a dead one on hand, take a look at her. Check gills (pink, swollen, black, etc), and check where the "rot" has hit - especially the body. If the body is sunken, it is exactly like Mystery Illness, which like I said, the mysterious rot eats away at living tissue leaving you with a live-dead fish that ends up dead in the end.
Maracyn 2 is more for fungus, API Furan-2 is anti-bacterial fr gram positive and negative bacteria... and Jungle parasite clear for internal AND external parasites. Wish I knew what it was. since it spread, it can be anything.
Have you fed any live foods previously, that only that tank gets fed?