Why did my cichlid die?

[Partridge27]

Hi, I came home today to find one of my cichlids dead. All of my water has been tested an is perfect. Though yesterday I found some rocks that were ok for the tank, so I scrubbed them and got all the dirt off them but one of them had faint black spots. I was not sure if this was part of the rock and I scrubbed and it didn't come of so I figured it was. Though this one was dead all of my other ones seemed to be swimming around happily, any help appreciated

 

[msumpter87]

We need some more info like size of tank, stocking of tank, and what was the exact reading of the water testing.