Two scenarios come to mind: Either a low oxygen event or a gill disease. Nothing else will kill 100% of the fish, yet leave all/most invertebrates alive.
Trouble is, now that the fish are deceased, there is no way to tell which issue that was. Low dissolved oxygen can be really tricky - your tank circulation is borderline o.k., I've seen cases where all it took was the person to top the tank up a bit higher than normal to shut off any surface agitation and kill the fish. However, low oxygen almost always happens overnight (due to reverse phase photosynthesis).
However, you noticed the fish were acting "off" before the lights went out. That could point more to a gill disease. The one that kills fish that quickly is marine velvet, Amyloodinium. Most often, the only symptom that you will see is rapid breathing, hovering the water current, followed by death.
If it was a disease, you should leave this tank fishless for 45 to 60 days.