If you have planned plants considered "heavy root feeders" then you shouldn't have to worry about it. These plants include Echinodorus sp, Cryptocoryne sp, Valisneria sp, Sagitaria sp and, some Hygrophila sp. But, even stem plants pull some oxygen down to their roots. Also, you can utilize MTS(Malaysian trumpet snails) for light aeration. They will dig through the top inch or so of substrate. If your substrate cap isn't too deep you really won't have to worry so much either.