Pungent Chemical Caused by Fish Tank Decomposition
Puzzle 12 of Aquarium Easy Pack
Question is: Pungent Chemical Caused by Fish Tank Decomposition
and the answer is: AMMONIA
Word Meaning:
AMMONIA — (inorganic chemistry) A gaseous, toxic compound of hydrogen and nitrogen, NH₃, with a pungent smell and taste.
Definitions from Wiktionary.