Hi, I'm working on a commissioned story, the basic jist of it is aliens colonize earth, begin a lengthy process of terraforming it to fit their toxic homeworld atmosphere. I read somewhere chlorine is the most viable alternative to oxygen in terms of energy production and it's fairly common in the universe and toxic to humans so I went with that. The flipside would be that oxygen would be toxic to the aliens, so it would have to be removed. How would one go about practically doing that? Depleting earth's oxygen supply and creating free elemental chlorine? It would have to be fairly closely based on existing earth technology as the aliens would be a small but powerful invading force unable to bring much of their own technology, instead adapting ours.