Simplify the following text for a Grade 9 native English-speaking person.
Original: In marine aquariums calcium chloride is added to introduce bioavailable calcium for calcium carbonate shelled animals such as mollusks and cnidarians . Calcium hydroxide ( kalkwasser mix ) or a calcium reactor can also be used to introduce calcium , however calcium chloride addition is the fastest method and has minimal impact on pH.
Simplified: Calcium chloride is a chemical compound made of calcium and chlorine atoms .