I have no idea what caused it. My ammonia and nitrites tested at a perfect 0, my nitrates tested at 7 my temp has been a beautiful 80. Perhaps it was the two new guppys I had added on Saturday (non qt'd

) maybe one of them had something wrong with it that i couldn't see. but they both acted very happy and healthy until i came home and found them dead. The others started looking a bit depressed on Sunday night and really bad on Monday morning, by Monday night all were gone but 'Lil Bit, who was made it until 3:00am Tuesday morning. My cory cats are all 3 very happy and healthy looking fish. So I am at a loss as to the exact nature of their deaths.