Moorish Idol - Take 2

[Samw]

Makes sense Brad. Your sump with the protein skimmer was highly oxygenated but that oxygenated water was not coming back into your main tank fast enough because of the reduced return flow. Night time deaths of the highest O2 demanding fish would result when DO levels drop at night. For most the easier to keep and less demanding fish, this would not be a problem.

If I turned off my skimmer tonight and only leave my powerheads and an aquaclear running, tomorrow I will wake up with dead dwarf angels but no problems with everything else in my tank including tangs. Happened every time.