What happened to my South American Cichlids?! HELP?

I had a very large column tank, in my tank I had 5 Blood Red Parrot Cichlids, a turquoise Severum, a jack dempsey and a convict cichlid. All of these fish I have had for years, so behavior is out of the question as well as tank size. I went away for two nights (mini vacation) and had my parents take care of the pets, the fish and my cat. I came home this morning only to find my tank full of black water and every fish is dead!!!! I'm furious and they have no idea what happened. If anyone could provide insight on what they think happened I would greatly appreciate it. The heater and filter were still functioning normally, however I have drained the tank and rinsed all the ornaments and gravel. I do not even want to begin putting a tank back together until I know what caused this. I've had cichlids all my life and have never seen anything like this