My opinion/experience:
It’s happened to me three times. 2 times when starting a new tank w/bacteria in a bottle (QT tanks) and once employing maintenance bacteria (Display tank).
It will clear on its own. Oxygen depletion is the chief danger, and since you have a sump/overflow I don’t think your livestock will suffer. You always worry when offering such opinions because you dont want to harm other people’s tanks, but I’ve never lost anything during these events.