Aquarium additives include calcium, iron, magnesium, water conditioners, kalkwasser, and phytoplankton. Many are necessary for the survival of the animals in a reef tank, while others will help to enhance the health of your tank's inhabitants.
There are many different manufacturers of aquarium additives. The most popular are Kent Marine, Seachem and E.S.V. The most commonly used additives are Calcium, Alkalinity, Iodine, and Phytoplankton. Popular additives are E.S.V.'s B-Ionic Calcium Buffer System, American Marine's Selcon, AZ-NO3 (for effective nitrate reduction), Boyd's Chemi-Pure, CaribSea's aragaMIGHT, and various other Kent Marine or Seachem additives.