If you plan to keep a full blown reef tank, here is what I recommend for additives: phytoplankton (for filter feeding corals), calcium, essential elements, strontium/molybdenum, iodine (if you want to have xenia excel and spread), Marc Weiss' Coral-Vite (for help in growth of coralline algae), and an 8.2 or 8.3 pH buffer (I prefer SeaChem's 8.3 buffer, which not only adjusts pH perfectly, but also alkalinity).
I like to use Kent products for my calcium, essential elements, iodine, and strontium. However, Kent puts out a coral vite too, but I definitely think Marc Weiss' works a lot better for growing coralline algae.
Another optional additive is iron/magnesium. It helps with the growth of algae, plus it is good for invertebrates. Most salt mixes have iron in its trace elements, but iron gets depleted very quickly, especially with precipitation. Kent's iron/magnesium puts iron, manganese, and potassium into your tank, so a lot of people like this too.
