I know this is after the fact, but FWIW, was the biowheel new and /or were any meds added that could wipe out the biological filter? Ammonia / nitrite burn on the gills could cause that behavior. Also, if the tank has a snug lid with the only opening being by the biowheel, I've seen set-ups with HOB filters have their oxygen level bottom out. The addition of an air stone / sponge filter quickly remedied the problem. In those cases, though, it happened within the first 24-48 hrs which doesn't match your situation, but if something temporarily slowed the flow of your biowheel, it's possible this could be a cause. I'd put more stock in the former than the latter, though. Just food for thought.