5 year tank crashed. Unable to reestablish.

[Emmet]

Hello.
A friend who has had a tank for the last 8 years without any deaths in the last 7. No spikes and completely stable. Suddenly the tank crashed and all of the fish died. He has had tanks the last 30 years and this has never happened before.

We have tried to reestablish the tank with secheme bacteria with no luck. Still extremely high ammonia.

The only reason I can see this being is because of the plant in the tank. I believe it is a Amazon sword species. A massive one at that.
Pictures of the tank and plants are pictured below.

 

[BlueJungle]

Hi, can you tell us what all of the water parameters are, please?

 

[Colin_T]

what fish were in the tank?
what symptoms did they have?
what did you do the day before they started dying?
have you washed the tank out and hosed the gravel out to remove any gunk?