omg everything dead in few days

[ALang]

Sure sounds like FW septicemia when I had FW fish!
Also, check your Ammonia again! Your testing fluids maybe off.

Not sure what can be done at this point. Fish that far gone cannot be helped. In the mean time, lots of water changes and carbon the heck out of the DT. Make sure you clean the equipment (nets, buckets, siphon tubes etc) so that you don't spread that around.

Sorry to hear about this.
Hate to see a tank completely gone from one fish. Good luck.