If you can't get your hands on one or more surface aerators I would rent a trash pump from your local rental supply and rig it to pump your suface water out onto the surface of the pond. You want to create as much spray and suface distrubance as possible. Diffusers are not really good at adding oxygen in emergency situations. Night time is when you should worry the most. From the literature I have seen 12 inches or less is the time to worry but it. Doesn't hurt to do something just in case you are headed for an algae crash.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.