Any info on pH, kh, gh, tds?
What substrate? Any rocks or anything that could be leeching anything harmful?
Any chance toxic chemicals are entering the tank on accident?
I've heard rare instances where decorations leeched things, but the water would cloud and they would give off a horrid smell.
The sole surviving rasbora, did it transfer any disease to any of the fish in the 55?
I've heard of Gourami's having a common disease, but not sure if it can infect other fish species as well. Any signs/symptoms of disease at all (visible indications or behavioral signs)?
I am thinking it has to be either disease or chemicals given how many die. Is kind of strange the lone rasbora would survive unaffected though, but hey you never know.