All Fish Dead

[Mount_Prion]

Electrocution, also are the bodies intact, did someone go on a killing spree and killed everybody (somebody fought back was killed and mortally wounded the killer)

This is also a good possibility. Could be that a heater's wire became exposed to the water or something.

 

[Loshmitchell101]

I had this happen to me one time my RTC threw up inside the tank mixed with cichlids, it was a little more then 12'' at the time in a 150g, it clouded the water pretty bad. What did the water look like when you found this was it super cloudy? When did you feed the RTC last?

 

[thebiggerthebetter]

Filter and bubbler shutting off are good possibilities, as in the electric power going down for some time and then restored but too late. As the OP said, he was not there for perhaps 8-12 h or more.

Electrocution does not fit, I don't think. The voltage is not high enough and either ground-fault circuit will disengage or a fuse will turn off instantaneously... before any significant damage occurs.