Do you notice your fish first thing in the morning OR in the heat of the day up at the top of the water 'gasping' or gulping...like they are eating small bugs?
If you do, I'd add air...if not, you probably have plenty of water movement for your small pond.
I was having problems first thing in the morning last summer with the gasping. Simply doing water changes wasn't helping in the 'still' tanks. I added an air pump because I didn't have enough electricity available for four more pumps and filters. The fish quit gasping. I ran the pump all winter and unless we had an exxtremely long period of freezing weather, there was always a hole open.