The point about oxygenation is actually a good one, and I wish I had thoguth of it!
I know its true because I remember from Microbiology lab, when we grew bugs that were anaerobic and would die if exposed to oxygen, we just put a 2" layer of oil on top of the broth and they were happy little buggers.
There is a way around this however...
Water is heavier than oil, and I don't think that goldfish have very high water O2 requirements. So, if they were to pour some fresh water in the tank every night after the friers were turned off, the water would sink and potentially provide enough oxygen to keep the fish alive.
Now that I have spent ENTIRELY too much thought and time on this subject, I think I'll study for my embryology test tomorrow...