Hi Jo!
Wow, there's a bit of a puzzle. I can only assume that the water is loaded with a toxin. Filter with charcoal/carbon. Before you do that, do a full spectrum test on the water for EVERYTHING to see what the problem is, exactly. If you don't have a full test kit, LFSes can usually help you out (for a small fee). Good luck and I hope you sort it before it's too late for the remaining fish.
As for the cloudy water, this is usually a typical algae bloom symptom. perhaps the tank is re-cycling?