In the wild, animals have cubic miles of seawater from which to draw the minerals that they need. In the enclosed environment of a marine aquarium, however, animals often suffer because they quickly use up available minerals.
However, essential minerals and trace elements are lost in protein skimming, filtration, ozone, reverse osmosis, de-ionization, pollutant absorption and other activities necessary to maintain your aquarium.
More minerals are used up by corals and other invertebrates. Topping off your tank with reverse osmosis, de-ionized or otherwise de-mineralized water will help to maintain clean water, but it will not help replace lost minerals. Therefore, mineral supplementation is needed.
Supplemental calcium is particularly important for corals as well as to help control pH swings. Tanks with heavy coral populations require additional strontium. Macro algae will require Iron supplementation. Other minerals may need to be added as necessary.
A more recent and very elegant way to supplement your reef tank is with the use of a Calcium Reactor. Calcium reactors automate the process of replenishing calcium and other trace minerals in your aquarium – minerals used by corals, invertebrates and even algae during their growth and life cycles. Sitting in the sump of your wet/dry filter, it is essentially a chamber filled with aragonite (crushed coral skeletons) that is infused with water and CO2. The CO2 lowers the pH of the chamber water (usually to about 6.7 or so) and dissolves the aragonite, reducing it to a solution of healthful minerals. The solution is more alkaline due to the addition of the dissolved minerals. That water solution is redirected back into the tank where it helps to stabilize the pH as well as replenishes the minerals for use.
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Additionally, live corals require strong lighting of proper spectrum and immaculately clean systems.