Help, im at a loss. 5 fish dead in less than 1 week

[Aquattro]

PO4 and NO3 won't kill fish, within reason. It sounds like some sort of contamination. Maybe something like a cucumber was in the rock and died during transport? Cleaners get in the tank?
Was this used rock from someone else? Maybe there was a problem with it? Did it have fish in that tank?
Are you running carbon?