If you're adding tap or spring water, you probably have plenty of aeration. If you're adding boiled water, you should probably invest in either an 02 set up, or an aquarium pump. When I did extract brewing, I added my top up water through the sprayer in my kitchen sink, and that added plenty of aeration.
When I was doing extract, I never needed an oxygenation system of any kind. When you top off the Carboy with water on top of your Wort, you have plenty of oxygen in the Carboy. The simpler, the better I always say.
Since going to AG I have not had a problem with splashing the wort into the carboy. But I have also been thinking of getting a pump, stone and filter. Sorry I'm not much help but would like to see more response to this thread. I know what I do works but could it be improved with aeration?