May I leave liquid in metal overnight? If no, what is the reason?
The Alter Rebbe in Shulchan Aruch brings down that one should not drink water that was left over night in metal. This is connected to spiritual “bad spirits” (“רוח רעה”).
The common custom is not to be particular about drinking water from the faucet, which has been in metal tanks over night. There are many justifications for this custom and they have what to rely on. Rumor has it that the Rebbe was not particular about drinking from a plugged in urn that had water over night.
Nevertheless, as a general rule, one should be careful where possible.
שו”ע אדה”ז הל’ שמירת גוף ונפש הל’ ז’. אג”ק ח”ב עמ’ קמ”ג. ראה גם שלחן מנחם יו”ד עמ’ כ”ב ואילך. גל’ התקשרות תרנה עמ’ 18.