http://www.fondriest.com/environment...solved-oxygen/
Also,
Not sure why but I almost never see people consider nor accept hypoxia as a cause of death when the same fish (even previously established ones) die consistently (overnight most likely) within a week while all others in the same tank are fine and there are no visible signs of disease and there is no aerator. The others who said Flames are easy had aerators. I think this happens more often than is reported.
http://www.aquariumpros.ca/forums/showthread.php?t=30777 "
What skimmer are you using? None"