When the lights go out, photosynthesis stops, which means that oxygen is no longer being given off into the water, and carbon dioxide is no longer being used up from it. You could have a funky live rock, or something going on in your sand that is using up more oxygen lately. Maybe a crab or a clam deep inside a rock died or something. You might not see a spike in nitrites or ammonia, but the decay still consumes oxygen. This could result in active fish with high metabolisms, like the chromis, asphyxiating in the night. Most night time fish deaths that aren't the work of obvious predation are caused by this. If the gills of the dead fish were flared out, this is almost certainly what happened.
Try and add more surface agitation, at least for a couple weeks, and look for anything that might have died out of sight. If it's down in a rock you won't find it, but maybe there's a chunk of food that your lobster missed somewhere.