My thought process here is that we have a bacterial disease within the fish with a secondary disease like epistylis. Bacterial diseases are fast killers in fish (compared to Ich or ammonia poisoning etc) so its a primary and secondary thing. The symptom of fast fish deaths (including some with no visible indication) makes me feel this is bacterial at its core. (Unless something else was done yesterday, which added to the problem).
Unfortunately, bacterial diseases are more challenging to treat than they are to prevent. Antibiotics have the highest success rate, but even they are ~50% effective. To add to this problem, we don't have an abundance of mature filtration to ease the load on the fish's immune system, which is making this even harder to stop. Even moving the 'healthy' fish to a different aquarium is a challenge as, as potentially evidenced overnight, we may not be able to see what fish is harbouring the bacterial disease.
I really wish I had a magic bullet here, but we are definitely within a nasty challenge.