As your reef tank ages, functions of some elements and compounds within the system begin to diminish. Major elements like calcium are consumed, as are many of the trace elements like iodine or iron. For this reason, some elements and trace elements need to be added to a reef tank on a regular basis. In a natural habitat, many elements and compounds exist at very low levels, and since the animals have almost constant exposure to water moving across them, it allows them to remove small amounts of elements constantly. This is not the case in a closed system, where there is a finite amount of trace elements present which can be rapidly depleted. In a closed system, protein skimming, chemical filtration, and water changes all cause the removal of some of these elements and compounds.

Before considering each element's role it is necessary to understand what is present in saltwater. Saltwater contains virtually all known elements. Those that are found in high concentrations - at least 1 part per million ( ppm) - are called major elements . This list includes sodium, magnesium, calcium, strontium, potassium, chlorine, sulfur, bromine, fluorine, carbonate, and boron. At a salinity of 35 parts per thousand (ppt) , there are about 35g of major elements in a kilogram of saltwater. The other elements that are present in seawater are found in very insignificant amounts and make up approximately 0.1% of the total dissolved solids. These elements have come to be known as trace elements and while they are only present in tiny amounts, they have still been found to be critical for the successful maintenance of life. These important trace elements include phosphorus, nitrogen, molybdenum, lithium, cobalt, silicon, iodine, iron, vanadium, copper and barium. Some of these trace elements, such as phosphorus and nitrogen, are crucial for the completion of biochemical processes. Others are important in that they are extremely toxic if they reach higher than desired levels.

Of the most common trace elements, the only ones that need to be supplemented are calcium, carbonate, strontium and occasionally magnesium. The one that we are usually most concerned with is calcium. In a reef tank, calcium can be consumed constantly by corals, clams, algae, sponges, and even fish. For this reason calcium needs to be monitored and added regularly. Carbonate is just as important as calcium. It acts as a carbon source for many animals, and also acts to buffer the water. Measuring and maintaining the levels of both these elements goes a long way toward maintaining a successful reef tank. Some ways to accomplish this task include usage of calcium reactors, kalkwasser, and combination supplements.

One element whose usefulness has been somewhat controversial is strontium. This element is involved in skeletal growth of stony corals and seems to play a role in coralline algae formation. So it needs to be added for strong coralline algae growth. It may also be used by corals as a replacement for calcium when calcium is insufficient to build new skeleton. Therefore, whether it is necessary is somewhat dependent on the adequacy of level of calcium.

As noted above, protein skimming can remove trace elements. If protein skimming is used or if there is a large number of invertebrates in the reef tank, then some form of minor trace element supplementation should be employed. Some of the minor trace elements that may be beneficial include iodine, iron, bromide, fluoride, and vanadium. The trace elements can be added in a mixed trace element supplement. Iodine, however, should be added as a separate trace element as it oxidizes more rapidly than others. The entire dose of either of these should not be given as a bolus once a week, but should be given in small portions two or three times a week.

Additives and trace elements may not seem to play a significant role in successful reef keeping, but even small things such as these can have a deleterious effect when they are neglected.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.