ok.
It probably isn't the CO2 anyway - if it has been 2 days, CO2 concentration would dissipate within 12 hours or so.
Lets go back to the red bodies on the fish. You said it looked like it was near the gills possibly?
Also, is there a lid on the tank? Has anyone been spraying any air freshener, or cleaning anything with glass cleaner in the room, or anything like that?
I think something had to have gone drastically wrong to lose that many fish that quickly.
Disease seems unlikely to me given the time frame here.