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

[Hightower]

no carbon. yes it was used rock from another healthy tank. He never had a problem with it to my knowledge.
Im not sure of die off during transport as it was less than 10min and remained wet.

If a cucumber was in the rock or something else and it died and caused that much poison, should there not have been an ammonia spike?