(Spoiler Alert) Oxygen Tanks

For those who have seen this film: I just finished watching it, and found it an excellent example of 1950's sci-fi. One thing bothered me though. When one of the men in the town is pulled from the lake, and the doctor inadvertently kills him by adminstering a little pure oxygen, once the doctor and the wife know that he was an alien like many men in the town, why didn't they simply use oxygen tanks to quickly kill all the aliens?