What really matters is the surface agitation. Bubblers will do that, but so will any pump as long as the outflow is directed toward the surface. A better idea,

What really matters is the surface agitation. Bubblers will do that, but so will any pump as long as the outflow is directed toward the surface. A better idea, I feel, is to just keep a layer of air between the water surface and the cover. The key issue is that oxygen has trouble crossing the air/water boundary, so you want that to be as big as possible. Meanwhile, if you keep the air holes in the cover small enough, most of the humid air will be trapped inside, and you will minimize losses from evaporation.