My instinct would be:
1. Something got in the water. Things I would be suspicious of are room fragrances, deodorant, body spray, window cleaner, furniture polish, febreez, that kind of thing. Is there anyone else in the house that could have done something like that while you were out? Doubt it was the carbon insert, but it is possible that the filter was contaminated with something else. Was it brand new or second hand?
2. Recently added fish introduced some kind of infection. You said your newest fish was added a week ago. I'm assuming you didn't quarantine? A quarantine period of 4-6 weeks is enough time for any disease to show itself - I know many people don't do this, but my point is that even though the fish has been fine for a week doesn't mean it is disease-free.
As Ian said in the other thread, if in doubt, water change. You don't need to wait between them, back to back is fine, and they should be almost total (ie 95%). If you suspect the filter, disconnect it.
Sorry for your losses
I know how devastating it is to lose your fish. Condolences and I hope the rest survive.