This Copper Reagent measures levels of copper. Proper copper levels can assist with ridding algae from the pool. However, high levels of copper can cause water discoloration. Maintain a safe and clean swimming environment with the Copper Reagent. Easy to follow instructions allow for hassle free testing. Taylor formulates its reagents to remain effective for at least one year, with only very few exceptions (molybdenum indicator in liquid form is one; after four months old it should be tested against a standard periodically). As a general precaution, replace all reagents more than one year old, or at the beginning of a new testing season.