Growing up near several chemical plants as a child... when he put his head in there... I actually got a chill, like when you see a bad accident about to happen... My son actually asked me why my face went white. I see a lot of shit on this OSHA Reddit but this one really got me.
For a few good reasons why please read: Cyanide Canary by Joseph Hilldorfer
Never, I mean NEVER, put your head (or any other part of your body) in a tank of any type no matter what size or length of duration in the tank without proper safety equipment.