As Yves said, the concentration of o2 in the water does have a saturation point when no more o2 will dissolve.
There are some fish that become ill in water with a high o2 saturation, but most fish, especially plecs and other fast-water fish will benefit.
There only real downside to an air curtain is the increased agitation of the water's surface dissapates dissolved Carbon Dioxide (co2) more quickly which although good for the fish, is bad for plants.