help!!!!!

[Samw]

Not sure if heat would be the cause since the fish died at night when the temp is not at its highest. If a fish looked completely healthy during the day and ends up dead in the morning, without any other info, low oxygen is a very good guess. In my tank the oxygen drop is huge overnight. I've also had angels die in the next morning after I unplugged my aerating device and forgot to plug it back in. In addition, not all skimmers are good at aerating water. The ones that are poor are closed loop ones (The ones that don't actively draw water from the tank into the skimmer and back out).