It certainly is worth investigating other potential causes. Although, I do find it suspect that this happened right when your skimmer died. Were your fish showing any signs of distress? Rubbing against rocks? Have you tested your water parameters as Grizz suggested? Some fish not eating?
Still, in my opinion, this is a high bio load to sustain without proper oxygenation for a period of time, especially with the only two powerheads not pointing at the surface for surface agitation. It doesn't take long for one or two fish to get stressed due to lower oxygen levels as some require more than others. Then they start gasping and breathing heavier and heavier depleting oxygen faster and faster. You multiply this by a few more fish and you could be in big trouble fast. Then your first few fish die and you pretty much have a meltdown
