Complete guide to the chemical elements of the marine aquarium

KH (carbonate hardness / alkalinity) is one of the most structuring parameters in a reef tank: it represents the water’s buffering capacity and, in practice, it drives the system’s chemical stability. When KH is consistent, the tank handles swings better and calcifying…

Conductivity (EC) is a physico-chemical indicator that reflects how well water can carry electrical current—so, essentially, the total amount of dissolved ions. In reef tanks it’s valuable because it tracks the overall “salt load” very closely: when EC moves, it’s rarely a…

pH tells you, plainly, whether the water is more acidic or more alkaline. In reef tanks it’s a real “comfort barometer”: it influences tons of chemical and biological reactions, and a well-held pH often makes the system more predictable—especially with demanding corals.

Salinity (PSU/PPT) isn’t “just another number”: it’s the total concentration of dissolved salts that defines seawater. In reef tanks it directly drives fish/invertebrate osmoregulation and sets the frame for all your chemistry. When it’s dialed in, everything reads clearer; when it drifts…

Boron is an essential macro-element for coral growth and cell membrane stabilization. Present in seawater mainly as boric acid, it supports many metabolic and photosynthetic processes in corals and biofilms. Boron also contributes—more modestly—to the carbonate system and pH stability. Without adequate…

Bromine is a halogen that’s often underestimated but extremely important for coral health and coloration. Stony corals use it to build colored chromoproteins and support skeletal structure, while zooxanthellae incorporate it into irreplaceable enzymes for photosynthesis. Some soft corals such as Dendronephthya…

Calcium (Ca) is a fundamental macro-element in reef aquaria: the raw material for coral calcification (SPS/LPS), invertebrate shells and coralline algae. Aim for a practical range of 400–440 mg/L (target ~420–430 mg/L) at 35 ppt salinity to replicate natural seawater and ensure…

Chloride (Cl⁻) is the major anion in seawater: it “carries” a large part of salinity and contributes to the tank’s overall electrical balance. In reefkeeping, you don’t target it to “feed” anything (it’s not limiting), but because it directly reflects salinity stability—and…

Fluorine, present in seawater as fluoride, plays a key role in the health of hard corals. This halogen actively supports skeletal calcification, strengthens the crystal structure of the skeleton, and contributes to natural tissue defense against parasites and bacterial infections. When the…

Magnesium (Mg) is one of the major pillars of seawater, even if it is often overshadowed by calcium and KH. In a reef tank, its most “useful” everyday role is helping to keep chemistry stable: when magnesium is properly set, calcium tends…

Potassium is an essential macro-element for coral metabolism, involved in skeletal growth, cellular functions, and nutrient conversion. It also acts as a fertilizer for zooxanthellae and contributes to the formation of chromoproteins, the pigments that give corals their color. Without stable potassium…

Sodium (Na) is one of the pillars of seawater: together with chloride, it makes up most of sea salt and gives seawater its conductivity, osmotic pressure, and a big part of its chemical “signature”. In reef tanks it’s a background parameter, but…

Strontium (Sr) is a naturally present macro-element in seawater, often a bit underestimated even though it closely follows organisms that build skeleton: stony corals, coralline algae, and more broadly anything depositing carbonate. It’s not a “magic growth button”, but when it’s lacking…

Sulfate (SO₄²⁻) is the most common form of sulfur in seawater: a major ion, highly present, and generally quite “stable” in a well-run reef tank. You don’t chase it for a spectacular effect, but because it’s part of the ionic balance that…

Sulfur is a quiet but essential pillar in reef tanks: in seawater it is mostly present as sulfate, a major ion that is very stable. It shows up indirectly in core building blocks of life (sulfur amino acids, cofactors…), and in the…

Nitrate (NO₃⁻) is the “end of the road” of the nitrogen cycle: after ammonia/ammonium and then nitrite, you end up with nitrate. In reef tanks it’s not a poison to eliminate at all costs — it’s mainly a nutrient that influences color…

Nitrite (NO₂⁻) is an intermediate in the nitrogen cycle: it appears when ammonia/ammonium is processed, and is then converted into nitrate. In a well-running reef aquarium it’s simple: nitrite should be undetectable or only a trace, because the “second step” (NO₂⁻ →…

Phosphate (PO₄³⁻) is one of the most influential nutrients in reef tanks — not because it “poisons” the system, but because it drives a big part of the biology. It feeds microbes, algae and zooxanthellae, and it’s part of life’s building blocks…

Phosphorus (P) is the “raw material” behind phosphates: it’s an essential nutrient for everything living in the tank. It’s built into energy molecules (ATP), cell membranes, and the backbone of DNA/RNA. In reefkeeping, it’s not just about algae: phosphorus also fuels biofilm…

Silicon (Si) has a slightly unfair reputation in reefing: as soon as we hear “silicates”, we think brown film and scraping glass. In reality it’s mainly a nutrient that decides which organisms take over—especially diatoms (the brown algae that build a silica…

Barium is a trace element involved in coral growth and the control of calcification within the skeleton. Its role is still partly unclear, but observations suggest that a balanced ratio between barium, calcium, and strontium supports steady growth and a strong skeletal…

Chromium is a metallic trace element present at ultra-low levels in seawater. In corals it mainly shows up as a cofactor in lipid metabolism and can be found in tiny amounts within the skeleton. At these very low levels, it may help…

Cobalt is an ultra-trace element, present at incredibly low levels in seawater, yet central to reef biology through vitamin B12. It doesn’t “feed” corals directly like macro-elements do, but supports bacteria, phytoplankton and zooxanthellae that rely on B12 to divide, fix carbon…

Copper (Cu) is the perfect example of a “double-edged” trace element in reefing: essential to many biological mechanisms (enzymes, cellular respiration, protection against oxidative stress), yet potentially dangerous as soon as it becomes too bioavailable. In a tank, copper is often partly…

Iodine is a key trace element in reef tanks. Even at low concentration, it helps protect corals from light stress, supports tissue function, and contributes to overall tank vitality. Corals, clams, crustaceans, and microfauna continuously use iodine for detoxifying excess oxygen, building…

Iron (Fe) is a “pillar” trace element in reef tanks: it supports living metabolism and plays a central role in essential enzymatic processes. In practice, you mostly notice it because well-managed iron can contribute to better overall vitality, with effects that are…

Lithium is a light alkali metal that is present in surprisingly high amounts in seawater compared to many other trace elements. Corals incorporate it into skeleton and tissues, likely in a mostly passive way, and no clear biological “must-have” role has been…

Manganese (Mn) is a quiet but valuable trace element in reef tanks: it supports enzymatic mechanisms and, in practice, it’s mostly known as a growth-supporting nutrient for coral vitality. It also shows up in the “color equation”: well-managed manganese can help keep…

Molybdenum is a key trace element in the nitrogen cycle. It’s a transition metal that acts as a cofactor for several critical enzymes—especially those that allow bacteria, algae, and zooxanthellae to use nitrate and, for some organisms, to fix atmospheric nitrogen. A…

Nickel (Ni) is a quiet but useful trace element: it supports enzymatic reactions in the tank’s living ecosystem, especially around assimilating certain nitrogen forms and the work of microorganisms that indirectly feed the reef food web. When balanced, it can support overall…

Rubidium is an alkali metal very close to potassium, present in small amounts in seawater and therefore in reef aquaria. Its biological role in corals isn’t clearly established, but it may be incorporated into skeleton and growth tips by partially “mimicking” potassium…

Selenium is an essential trace element for corals, especially in strongly lit tanks. It’s part of selenoproteins involved in antioxidant defenses and helps protect the cell membranes of corals and their zooxanthellae. When correctly balanced, it improves tolerance to intense light, supports…

In reef tanks, silicon is mostly encountered via silicates (from silicic acid). It has a “scary” reputation because it can feed diatoms… but it’s not simply an “enemy”. In a living tank, a small amount of silicon can also support organisms that…

Vanadium is an essential trace element in the transition-metal family, relatively abundant in seawater and involved in many subtle physiological processes in reef systems. It plays a role in enzyme systems linked to halogen transformations, nitrogen metabolism, and the activity of certain…

Zinc (Zn) is a “small but mighty” trace element in a reef tank. It acts as a cofactor for many enzymes and supports both healthy coral tissue and strong biofilms (the bacterial life that keeps the tank running). When it’s well balanced…

Aluminum is a common pollutant in reef aquariums, even though it naturally occurs in seawater only at very low levels. Unlike essential elements, it has no known biological role in corals. Excess aluminum often comes from filter media, additives, certain foods, or…

Antimony is a potentially toxic metalloid with no known biological role in reef aquariums. Its presence usually comes from external contamination: low-quality plastics, degraded PVC plumbing, cement-based decorations, or certain frozen foods. While some bacteria can metabolize it to a limited extent…

Arsenic is a toxic metalloid with no biological role in a reef aquarium. Its presence mainly results from external contamination: artificial decorations, glues and adhesives, poor-quality foods (especially those made with rice flour), contaminated frozen foods, or regenerated adsorbents. Unlike essential trace…

Beryllium is a toxic element with no biological role in reef aquariums. Its presence points to external contamination with poorly identified sources, although some frozen foods—especially brine shrimp (artemia)—have shown elevated levels in certain analyses. Beryllium can bioaccumulate, and while its exact…

Cadmium is a toxic heavy metal with no beneficial role in reef aquariums. While a few diatoms can use cadmium in a specific enzyme and some corals (such as Goniopora) may accumulate it in their tissues, cadmium disrupts key enzyme formation by…

Cesium is a trace element naturally present in seawater at tiny concentrations. In reef tanks it’s not known to “boost growth” or “improve color”: its value is mainly informational, because it largely reflects what’s in the water around your animals.

Gallium is an extremely rare trace element in seawater, present at such tiny levels that it’s not part of the typical reef “toolbox”. If it shows up on an ICP, it’s usually just a background trace… or a potential hint of a…

Lanthanum (La) is a bit of a special case in reef tanks: it’s not a “nutrient” that corals need, but rather an element you may see either as a natural trace or because it was introduced through certain water-treatment approaches. Where it…

Lead (Pb) is an undesirable heavy metal in reef tanks. It brings nothing positive to the system, and its value in an ICP test is mainly “diagnostic”: when it shows up, it almost always points to pollution (source water, materials, dust, certain…

Mercury (Hg) is a heavy metal that has no “good side” in reefkeeping: it does nothing beneficial for the tank’s life and is feared because it can be toxic even at very low doses. If it shows up in an analysis, treat…

Neodymium (Nd) is a rare earth (lanthanide). In reef tanks it’s not something you “optimize”: we care about it mostly because detectable levels above background can be a contamination marker, and one very common culprit is… magnets (pumps, mounts, scrapers) when their…

Silver has no beneficial biological role in a reef aquarium and must be considered a hazardous contaminant. Its presence most often results from using anti-cyanobacteria products containing colloidal silver, whose effectiveness is questionable and whose side effects can be catastrophic. Silver has…

Tellurium (Te) is a very rare element that belongs more to “technical traces” than to reef parameters you actively manage. It has no documented biological usefulness for corals, fish, or invertebrates, and when it shows up on an analysis it looks far…

Thallium (Tl) is one of those heavy metals you want to see… nowhere in a reef tank. It brings nothing positive to livestock, and its main value on an analysis is as a pollution marker: if it shows up, think “unwanted input”…

Tin (Sn) is one of those metals you don’t try to “balance” in reefkeeping: if it shows up on an ICP, it’s mainly a contamination marker. At low levels it may go unnoticed, but when it rises it can become genuinely troublesome—especially…

Titanium (Ti) is a bit of a special case in reef tanks: you mostly run into it because it’s a material (alloys, mechanical parts, probes), not because it’s a “parameter” you try to maintain. On lab results it’s mainly a trace indicator…

Tungsten (W) is an element you mostly encounter through ICP testing, often sitting somewhere between “trace” and “pollutant” because in reef tanks it has no clear role for corals, fish, or invertebrates. In seawater it exists at very low trace levels and…

Uranium (U) is naturally present in seawater. In reef tanks it’s not a “trace you optimize”: it’s better seen as a background marker that follows the composition of salt, rocks and the water you use. At natural levels it is generally considered…

Zirconium (Zr) shows up in reef tanks mostly for a very “material” reason: it’s used in ceramic components (notably some pump shafts/bushings). For livestock, it has no known biological role in corals, fish, or invertebrates, which is why it’s often grouped with…