I found this information, it answers numbers 1 and 2
Hydrogen Sulfide
Hydrogen sulfide forms when bacteria feed off decayed organic matter and is trapped under the sludge layer. Hydrogen sulfide produces a rotten egg odor, and gas bubbles may escape when the sludge is stirred up. These gas bubbles can also release carbon dioxide, methane, and other noxious gasses, lowering the level of oxygen present. Hydrogen sulfide is toxic to fish, especially in warmer water temperatures
Here is a link to more info it is pretty informative.
http://books.google.com/books?id=Sz4RYEy...num=3#PPA144,M1
