I am not aware of any anurans with external gills. You need to make sure that what you have is a frog and not a salmander. Tadpoles have external gills and I recommend mild aeration to oxygenate the water, water exchanges serves other purposes and is not ideal for oxygenation.
:Okay, so my wood frog taddies had frilly little gills for a couple days after they hatched. Now the gills are gone from them. So - how are they breathing? I keep reading about regularly changing the water out so it's oxygenated - but how are they breathing after they lose their visible gills? I don't see them gulping at the surface.