Additives are trace elements that are added back into the tank as the inhabitants consume them over time. The necessity of additives is hotly debated because solid scientific proof of their necessity is somewhat lacking at this time. It is easily shown that certain elements such as strontium are taken in by stony corals, but automatically assuming it is necessary is spurrious. All corals take in cyanide when present in the water, and the last time I checked, nobody is championing the addition of cyanide into our tanks! Furthermore, necessity to add particular elements is diminished when you consider that regular water changes replenish many of these trace elements. If you feel you must dose trace elements, try and test your water occasionally to make certain your levels are not above what the levels found in nature are. Even though the need for iodide, magnesium, strontium, and many other elements are still in question, I would venture to say that calcium and the maintenance of high alkalinity is necessary for the health of a reef habitat.