Magnesium
When it comes to reef tank water chemistry, magnesium is easily the most overlooked element. Without the proper amounts of magnesium it will be impossible to hold the correct amounts of calcium and alkalinity. Many reefers only adjust their magnesium level once a week to monthly, but you can bet some of the most successful reef keepers out there keep an eye on their magnesium and will always ensure it is at the proper levels.
