Additives make up a group of elements essential to keeping a stable marine aquarium.  Research animal health requirements to narrow down the needs of your set up.  Then keep a supply of additives on hand and dose as needed.  There are monitoring systems as an alternative to daily or weekly water testing, such as the Neptune Monitoring System that will notify you when a certain additive is needed.  Major advancements have been made in the technology that helps you know when and what to add.  Stable chemistry is vital to every environment.