Depending on your setup, I dont believe covering your sump would have any ill effects on heating or gas exchange. First of all, it in no way will be air tight, so there will still be oxygen among other gases inside of the lid. The water pouring out into the sump should oxygenate the water enough that it should not matter. Also, if it is a sump only, there will not be a light on it most likely, so there should be no added heat from the top being on. I have my sump ( 100g) and display ( 220g) covered by glass tops and I have no problems in my tank, either with heat, or any signs displaying that the oxygen in the tank is low. If worse comes to worse, you could add a little air pump and a stone in the inlet side of the sump to aerate the water, although I doubt it would need it. This would get rid of the bubbles through the bubble trap, and you should see no microbubbles in the display if the sump is set up correctly.