Axies that constantly go up to the waters edge to get air means you dont have enough air in the water. You need to add a air stone on a low setting this will the move the op of the water which in turn uts more air into the water.
All 3 of my axies do go up to the surface from time to time for a gulp of air, this is normal.
When a axie is flicking its gills it is pushing fresh water over them so to get more oxygen, not to make oxygen.
As far as im aware a morphed axie (which is rare) will breath with its lungs, as during metamorphosis they develop so they can use them for brething, Mikki or Mike, please correct me if im wrong.
So if they are coming to the surface quite alot add a air pump and stone... you will soon see less of it.