Trace elements, macro-nutrients and micro-nutrients in the reef aquarium

One result, thirty elements

The first reaction when looking at a laboratory analysis printout is often bewilderment. The page contains dozens of figures, reference ranges, ratios and graphs. One value is elevated, another is below the reference range, selenium doesn’t appear at all. How should you approach this?

Before paying attention to individual figures, it is important to understand what the result tells you as a whole — and what it does not. An ICP-OES analysis (inductively coupled plasma, optical emission spectrometry) is a snapshot of the chemical state of the tank’s water volume at that moment. It does not say why a value is where it is, nor does it say what to do next without context: the tank’s history, coral population, dosing method, feeding and water change schedule all affect how the result is interpreted. Laboratory tests are covered in more detail in their own article.

This article uses the ICP-OES Total test as an example — a more comprehensive package than basic ICP-OES, covering macroelements, dynamic elements and the relevance line. It is important to understand the method’s limitations: ICP-OES does not detect all ultra-trace elements reliably. Rubidium and some other relevance line elements require the ICP-MS method (mass spectrometry), which is considerably more sensitive. This is why some element results are unreliable or missing entirely in an ICP-OES report.

This article covers all the ICP-OES Total element groups — macroelements and halogens, dynamic elements and the relevance line — and explains what each element does, why it is measured and which direction it should be in.

The biochemistry of calcium, magnesium and alkalinity is covered in more detail in the articles The big three in practice and Calcium basics. The dynamics of nitrate and phosphate are also covered in their own articles.

Dividing elements into three levels

Substances dissolved in seawater fall into three practical categories based on their concentration. The division is not scientifically absolute — it is a hobbyist’s tool that indicates the unit of measurement involved and how readily a value changes in a closed tank.

Macroelements appear in seawater at concentrations above 1 mg/l. There are many of them, they are consumed in measurable amounts and their change is visible even in home test kits. This category includes calcium, magnesium, potassium, sodium, chloride, sulfur, bromine, strontium, boron and fluorine.

Dynamic elements, or trace elements, appear at concentrations below 1 mg/l — typically at the µg/l level, meaning thousandths of what macroelements measure at. They are biologically highly active: cofactors for enzymes, building blocks for colour pigments, components of the immune system. Even a small change in concentration can be visible in corals quickly. This category includes zinc, vanadium, copper, nickel, manganese, molybdenum, iron, chromium and cobalt — as well as iodine, which is a halogen but behaves in terms of concentration like this group.

The relevance line covers ultra-trace elements and potentially harmful substances. Some have a biological function (lithium, barium, selenium), some are indicators of contamination sources (lead, cadmium, mercury). These are measured primarily so that what is in the tank is known — not because they are actively dosed.

Natural seawater as a reference point

All reference values are related to concentrations found in natural seawater (NSW). A reef aquarium is a closed system, but its inhabitants have been shaped by evolution for open ocean conditions. Deviations from NSW values are not automatically problems — but they always require a justification.

An ICP-OES Total analysis reports, alongside the measured value of each element, a reference range and a corrected value normalised to a salinity of 35 ppt. This is a practical addition: if the tank’s salinity deviates slightly from the target, most element values scale with it.

Macroelements and halogens

Calcium (Ca) — NSW 410–420 mg/l, target 400–440 mg/l

Calcium is the fundamental building block of the skeleton. Corals combine calcium with carbonates to form calcium carbonate, which crystallises in aragonite form to create the skeleton. Calcium is directly linked to alkalinity and magnesium — they form one chemical system that cannot be examined in isolation.

Too low calcium slows growth and weakens colour — the first symptoms appear in growth tips and polyp extension. Below 350 mg/l all corals begin to suffer clearly. Too high calcium combined with high alkalinity leads to spontaneous precipitation.

Indicator species: plating Montipora, Stylophora pistillata, Echinopora lamellosa.

Magnesium (Mg) — NSW 1,290–1,350 mg/l, target 1,200–1,350 mg/l

Magnesium is the balancer of the calcium system: it keeps calcium dissolved in water by preventing the spontaneous crystallisation of calcium carbonate. A stable calcium value is not possible without sufficient magnesium — this is not a metaphor but chemistry.

Below 1,100 mg/l: calcium and alkalinity values become unstable, LPS corals show tissue detachment from the base of the skeleton, coralline algae ring-shaped dissolution. Above 1,600 mg/l: tissue dissolution in soft corals, tissue detachment in SPS corals — this should not be pursued even for algae control purposes.

Alkalinity (KH) — NSW 6.5–7.5 °dKH, target 6.5–8.5 °dKH

Alkalinity — in practice carbonate hardness — is the third pillar of the calcium-magnesium system. It describes the water’s ability to buffer pH and is simultaneously the direct building material for coral calcification: corals combine calcium ions with bicarbonate and carbonate ions to form aragonite skeleton.

The target range for alkalinity depends clearly on tank type. SPS-dominated tanks typically maintain a lower level of 6.5–7.5 °dKH, because higher alkalinity combined with high pH increases precipitation risk and can cause RTN symptoms in fast-growing species. 7.5–8.5 °dKH is a functional range for LPS and soft coral dominated tanks. Stability is more important than the exact target value — daily variation above 0.5 °dKH is already stressful for corals.

Alkalinity is consumed in proportion to calcium during calcification: roughly 0.14 mmol/l of alkalinity is consumed per 20 mg/l of calcium consumption. If alkalinity is consumed faster or slower than calcium, this indicates unbalanced dosing or precipitation in the system.

The relationship between alkalinity and pH is direct: high alkalinity raises pH, low alkalinity lowers it. For this reason alkalinity and pH must be monitored together.

Indicator species: fast-growing Acropora species react first to both deficiency and excessive variation. Alkalinity deficiency appears as bleaching of growth tips and arrested growth. Alkalinity management is covered in detail in the alkalinity article.

Potassium (K) — NSW 380 mg/l, target 380–420 mg/l

Potassium is close to calcium in quantity among ions dissolved in seawater — but it receives considerably less attention. It is an essential macroelement for cell function, nutrient metabolism and zooxanthellae growth.

Potassium should be kept slightly below the calcium value (approximately Ca value minus 20 mg/l). A 5 percent drop from the reference value can already be visible in growth and colour. Deficiency leads to a grey, flat appearance, slowed growth and impaired nutrient processing — which can manifest as elevated nitrate or phosphate values without an obvious cause.

In Euphyllia species, sudden tissue dissolution can be related to potassium deficiency especially if phosphate values are low. In Seriatopora species, tissue breaks down from the base of the skeleton. Indicator species: Acropora valida, red plating Montipora.

Potassium rises readily in heavily fed tanks — it is released from organic matter during decomposition. Elevated values are corrected with water changes.

Boron (B) — NSW 4.5 mg/l, target 4–5 mg/l (SPS 5–6 mg/l)

Boron is an essential macroelement that occurs in seawater as boric acid. It participates in cell membrane stabilisation, growth processes and to a small extent in the carbonate system. High boron concentration also dampens the effects of excess aluminium.

For the SPS hobbyist: boron directly affects the metallic quality and lustre of coral colour. Bright colours and metallic sheen are difficult to achieve without a boron concentration above 4 mg/l. Below 2 mg/l → blister-like tissue detachment in fast-growing species.

Boron cannot be measured with a home test kit — an ICP laboratory test is the only way to verify the level.

Bromine (Br) — NSW 65 mg/l, target 60–70 mg/l

Bromine is a halogen and the seventh most abundant ion in seawater — but it is one of the most neglected elements among reef hobbyists.

Stony corals use bromine for chromoprotein synthesis: these are the same pigments that produce blue, violet and green fluorescence. Zooxanthellae need bromine for the production of enzymes involved in photosynthesis. Soft corals such as Dendronephthya species accumulate bromine in their tissue for toxin production. Gorgonians also accumulate bromine.

Below 50 mg/l: blue and violet fluorescence weakens first, colours become turbid, polyp extension decreases, contrast disappears. Above 90 mg/l: tissue detachment in the central part of corals.

Daily consumption is approximately 0.7–1.2 mg per 100 litres. GFO and aluminium-based phosphate removers also remove halogens from the water — bromine, fluorides and iodine. If a phosphate remover is in use, active ICP monitoring of halogen values is essential.

Indicator species: Cespitularia and blue Xenia species as well as gorgonians lose colour before changes are visible in stony corals.

Strontium (Sr) — NSW 8 mg/l, target 7–10 mg/l

Strontium is an alkaline earth metal like calcium, and it behaves chemically similarly — it integrates into the coral skeleton alongside calcium, making it denser and mechanically stronger. Although strontium is not considered directly essential in the research literature, its effect is clear in practical aquaristic experience: vibrant metallic coral colouration requires the correct strontium level. Strontium is also linked to coral immune function.

Deficiency → loss of growth and colour especially in coralline algae and stony corals, increased susceptibility to parasites. Strontium consumption is proportional to calcium consumption, so it is dosed in practice alongside calcium dosing.

Fluorine (F) — NSW 1.3 mg/l, target 1.2–1.5 mg/l

Fluorine is an essential element (importance level 6/6) — yet it is often overlooked. It cannot be measured with standard ICP-OES equipment and requires a separate IC/ISE HSA measurement. This is why fluoride analysis is missing from many laboratories.

The F:I ratio should be approximately 25:1 — fluorine at about 25 times the iodine concentration. This ratio is diagnostically more important than either absolute value alone.

Deficiency appears as turbidity, loss of colour at growth edges, increased susceptibility to parasites and partial light sensitivity. Below 0.8 mg/l, STN symptoms have been observed in some SPS corals starting from the base of the skeleton. Fluorine also affects blue colouration in many corals.

Aluminium-based phosphate removers actively remove fluorides — the same phenomenon as with bromine and iodine.

Indicator species: Acropora tenuis, blue Montipora species with a blue edge.

Iodine (I) — NSW 60 µg/l, target 55–80 µg/l

Although iodine is a halogen like bromine and fluorine, it operates at the µg/l level and behaves in terms of concentration like the dynamic elements. ICP-OES measures total iodine (iodate + iodide + organic forms combined).

In natural seawater, most iodine is in iodate form — the oxidised, more stable form. Ozone and UV sterilisers oxidise iodine rapidly to iodate, reducing the proportion of the most biologically active forms.

The significance of iodine is multifaceted: tissue health (deficiency appears as turbidity, loss of colour at growth edges), colour (blue and green colour weaken; iodine deficiency increases coral sensitivity to light), parasite resistance (low iodine increases susceptibility to e.g. AEFW flatworms and Montipora nudibranchs), and dinoflagellates (iodine dropping below 0.05 mg/l is one of the triggering factors for cyanobacteria and dinoflagellate problems).

Overdose above 150 µg/l → darkening of corals and vigorous algae growth. GFO and phosphate removers remove iodine from the water.

Indicator species: Acropora tenuis, blue and green Montipora species with a blue edge.

Dynamic elements — trace elements

Dynamic elements are the most biologically active elements relative to their concentration. They are present in the water at only micrograms per litre, but their effect on coral colour, growth and health is considerable.

A critical principle: dynamic elements do not operate in isolation but in relation to each other. Copper, which would be toxic alone above 20 µg/l, can be completely harmless if the other dynamic elements — zinc, vanadium, nickel and molybdenum — are in the correct proportion. Examining a single element is not enough: the entire dynamic element profile must be read as a whole.

Iron (Fe) — NSW a few ng/l, target 0.05–2.5 µg/l

Iron is a fundamental trace element for all life forms — it is involved in oxygen transport, electron transfer chains and countless enzymatic reaction steps. In natural seawater there are only a few nanograms of iron per litre: iron is the limiting factor for reef species in the open ocean, not nitrate or phosphate.

The biological effect of iron is most clearly visible in coral colours: green and yellow colour react first to iron deficiency. Acropora tumida-type green species fade to pale green or yellowish when iron is too scarce. At an appropriate iron level, the same coral shines bright green. Deficiency is also visible in Hydnophora and many LPS species as reduced polyp extension.

Excess iron behaves like fertiliser: corals darken, algae grow and the overall picture looks murky. Too much iron leads to an unstable situation especially if nickel and zinc are simultaneously low — nitrogen processing is then disrupted and nitrate can start to rise.

Zeolite systems actively remove iron from the water — in these tanks separate iron dosing is often necessary.

Indicator species: green and yellow Acropora species, Hydnophora.

Manganese (Mn) — NSW below detection limit, target 0.1–0.2 µg/l

Manganese is practically undetectable in natural seawater — it precipitates quickly. Biologically it is nonetheless essential: the manganese-calcium cluster is responsible for oxidising water to oxygen in the photosynthesis reaction. It is thus directly involved in the zooxanthellae photosynthesis chain.

Deficiency appears first as weakening of red and blue colour. In Goniopora and Alveopora species, turbid tissue, reduced polyp extension and light sensitivity are clear signs. LPS corals respond to manganese deficiency with reduced polyp extension.

Excess manganese darkens corals, increases algae growth and at worst can promote the formation of cyanobacteria. An important note: in many tanks manganese enters in excess from food — particularly from frozen fish, artemia and pellet foods.

Indicator species: green, blue and red Goniopora, Alveopora species.

Zinc (Zn) — target 3–8 µg/l

Zinc is essential for numerous metalloenzymes — particularly those involved in DNA replication, protein synthesis and oxidative stress management. Coral immune defence needs zinc.

Combined zinc, nickel and iron deficiency prevents corals and biofilms from processing nitrogen compounds normally — nitrate and ammonium can start to rise without an obvious cause. Zinc deficiency is also linked to STN symptomology.

Vanadium (V) — NSW 1–2 µg/l, target 2–10 µg/l (optimal 5–8 µg/l)

Vanadium is the second most abundant transition metal in seawater after molybdenum. It is essential for marine tunicates and sponges, and acts as an enzyme co-metal more broadly. In the reef aquarium, vanadium is important in the formation of colour pigments, photosynthesis processes and biofilm function — particularly in nitrogen-cycling bacteria.

The vanadium value can be maintained higher than NSW level without harm. Deficiency appears as: colour becomes monotonous, red hues fade, LPS corals lose contrast, in Acropora species deficiency can look like grey or spongy contraction at tissue edges. Particularly in LPS corals, vanadium prevents post-recession bleaching.

Above 30–50 µg/l vanadium starts to precipitate onto tank surfaces. Vanadium and molybdenum work together — the V:Mo ratio should be approximately 1:3–1:4.

Note: ceramic frag plugs can release vanadium into the water — new ceramic plugs are recommended to be soaked in RO/DI water before use.

Indicator species: spongy grey Acropora species, Stylophora pistillata without silver sheen, growing sponges and coralline algae growth.

Molybdenum (Mo) — NSW 10 µg/l, target 10–20 µg/l (optimal 12–15 µg/l)

Molybdenum is the essential cofactor of the nitrogenase enzyme — without molybdenum, nitrogen-fixing bacteria (diazotrophs) cannot function. Additionally, molybdenum is part of enzymes that reduce nitrate to ammonium in coral tissue. Molybdenum deficiency therefore inevitably leads to weakened growth and rising nutrient values.

An important property: molybdenum regulates the toxicity of copper to invertebrates. At the correct molybdenum concentration, a moderately elevated copper level is better tolerated.

Molybdenum also prevents the sudden “shifting” of corals — rapid change in colour and tissue state in a stress situation is often a sign of molybdenum deficiency. This applies particularly during changes to tank lighting or flow.

Molybdenum does not leave the water easily — an elevated value corrects only slowly through water changes.

Copper (Cu) — NSW below 3 µg/l, target 2–6 µg/l

Copper is a special case: it is a biologically essential trace element (enzymes, respiratory chain), but free copper is toxic even at small concentrations. In the aquarium, however, copper exists primarily in complexed form — bound to surfaces and organic compounds — where it is not bioactive and therefore less toxic than generally assumed.

Copper values up to 10 µg/l are harmless if the other dynamic elements are in the correct proportion. Without this balance, copper at 20 µg/l already causes bleaching first in Acropora species, then in Seriatopora and Pocillopora species, soft corals, molluscs and shrimp.

Sources of copper in the tank: water piped through copper pipes, sealants, metal parts, some salt mixes and residue from copper treatment used in quarantine. Copper binds to the surfaces of live rock and coral bases — it does not leave easily. An ICP value above 20 µg/l is a serious warning sign.

Nickel (Ni) — target 3–6 µg/l

Nickel is an essential element that improves growth and enhances red and turquoise-green colour. Nickel is particularly important for basal disc formation — if Acropora millepora is not forming a basal disc, nickel level is the first thing worth checking.

Deficiency disrupts nitrogen compound breakdown, weakens growth and increases susceptibility to parasites. Too much nickel → bleaching and partial peeling of tissue, reduced growth.

Indicator species: Acropora millepora.

Cobalt (Co) — target 0.02–1.9 µg/l

Cobalt is an essential element and the central building block of vitamin B12 (cobalamin). Symbiotic bacteria produce it in the mucus secretions of corals, but it also comes from feeding and dosing systems.

Cobalt is typically so sparse in the tank that it falls below the ICP detection limit — this does not mean deficiency, as dosing systems and nutrition normally cover the need.

Too much cobalt → increased cyanobacterial growth, darkening of corals. High B12 also promotes the proliferation of certain dinoflagellate species.

Deficiency → poor growth, turbid colours, low lustre, susceptibility to RTN or STN symptoms.

Chromium (Cr) — target 0.05–2.3 µg/l

Chromium participates in pigment metabolism. Deficiency appears as colour weakening, excessive value — as with most metals — as darkening.

The relevance line — ultra-trace elements and harmful substances

Lithium (Li) — target 180–350 µg/l

Lithium enters the tank primarily through salt mixes. It has no known active biological role in the reef aquarium, but it is part of the composition of natural seawater.

Barium (Ba) — NSW 5–20 µg/l, target 5–50 µg/l

Barium is an essential element for regulating calcification in the coral skeleton and should be in the correct proportion to calcium and strontium. Barium enters the tank from activated carbon, salt mixes, cement and coral adhesives and from food — separate dosing is generally not required.

Above 200 µg/l → tissue turns grey, especially if iodine is simultaneously low. Phosphate adsorbents (aluminium-based) can remove barium.

Selenium (Se) — target 0.9–5.5 µg/l

Selenium is one of the most important essential trace elements particularly for SPS corals. It is the part of the amino acid selenocysteine in the active centre of the glutathione peroxidase enzyme — in practice one of the most important antioxidant enzymes for protecting against cellular stress.

Above 20 µg/l → partial tissue damage and tissue detachment. Too low → growth disturbance, weakening of light tolerance, increased tissue transparency.

An important limitation: selenium may not appear in ICP-OES results even if present, because the concentration may fall below the detection limit. ICP-MS detects selenium more reliably. Before starting direct dosing it is advisable to seek expert advice — selenium overdose is quickly toxic.

Rubidium (Rb) — not reliably measurable by ICP-OES

Rubidium requires the ICP-MS method for reliable measurement — ICP-OES measurement quality is too poor for this element. The Rb value appearing in an ICP-OES Total report is at best indicative. High Rb values together with barium and caesium values are a strong indicator of zeolite use in the tank.

The effect of rubidium is interesting particularly in connection with Discosoma mushroom corals — especially hobbyists keeping so-called “Bounce Mushroom” morphs have reported benefits.

Harmful substances

An ICP-OES Total analysis measures a range of potentially harmful substances: lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), antimony (Sb). These should be at unmeasurable levels (n.d.). If any of these rises to detectable, it means there is a contamination source — typically in the plumbing, water supply, salt mix or handling equipment.

Lanthanum (La) — reference range 2–10 µg/l. An elevated lanthanum value is almost always a sign of lanthanum-based phosphate remover use.

Aluminium (Al) — reference range 5–30 µg/l. Typically comes from aluminium-based phosphate removers. Boron dampens the effects of aluminium.

Element ratios — more important than absolute values

An ICP-OES Total analysis prints out, in addition to absolute values, ratios that indicate the mutual balance of elements. These are in many cases diagnostically more important than individual figures.

The salinity line

The relationship of macroelements to salinity tells whether an element is at NSW level or at a deviating level relative to others. If calcium is high but salinity is normal, it means active overdosing. If both calcium and salinity are low, it could be dilution or measurement error.

The significance of element ratios

A practical example: if the iodine value is within the reference range but the F:I ratio is skewed, it means fluorine is at an abnormal level — perhaps due to phosphate remover. The absolute values look normal, but the ratio reveals the problem.

Predictive patterns: when element deficiencies lead to problems

Research based on extensive laboratory analysis data has identified element combinations that recur before certain problems. These are not individual cause-and-effect relationships, but combined effects:

Cyanobacteria: high NO₃:PO₄ ratio (target approx. 100, problem above 200) combined with iodine below 0.065 mg/l or bromine below 70 mg/l

Dinoflagellates: the same high NO₃:PO₄ ratio + iodine below 0.05 mg/l + bromine below 55 mg/l + molybdenum below 10 µg/l — this is covered more broadly in the dinoflagellate article

Parasites: iodine + fluorine + bromine + strontium + zinc + nickel all at the lower limit or below

Filament algae: high NO₃:PO₄ ratio + iron above 1 µg/l + manganese above 1 µg/l

RTN: iodine below 0.05 mg/l + fluorine below 1 mg/l + vanadium below 2 µg/l + nickel below 2 µg/l + sulfur below 700 mg/l + boron below 4.5 mg/l

STN: zinc below 5 µg/l + nickel below 5 µg/l + KH:PO₄ ratio clearly deviating (target approx. 200, e.g. 10 dKH / 0.05 mg/l PO₄)

Reading an ICP result in practice

Trend is more important than a single data point

One ICP result is valuable, but two consecutive results are many times more valuable — they indicate direction. Three or more already tell a trend. If manganese rises round after round, the problem is systematic rather than random. The laboratory tests article covers ICP frequency and test selection.

What to do when a value is low

Water changes are routine maintenance, not a corrective measure. A water change can lower values that are too high, but deficient elements cannot be raised effectively by water changes alone unless the salt mix contains that element in sufficient amounts.

When a value is low: check the salt mix composition (does it contain the element at reference concentrations), check whether any filter (GFO, activated carbon, phosphate remover) is actively removing the element, and assess consumption — a large coral population consumes more than a small one.

What to do when a value is high

For most elements, a high value is corrected by slowing or stopping dosing and performing water changes. Exceptions: molybdenum drops slowly, copper does not leave easily once it has bound to surfaces.

When any relevance line harmful substance is detectable (Pb, Cd, Hg), the task is to find the contamination source — not to adjust dosing.

References

Peer-reviewed research

Falkowski, P. G., Barber, R. T. & Smetacek, V. (1998). Biogeochemical controls and feedbacks on ocean primary production. Science, 281(5374), 200–206. https://doi.org/10.1126/science.281.5374.200

Ferrier-Pagès, C., Witting, J., Tambutté, E. & Sebens, K. P. (2003). Effect of natural zooplankton feeding on the tissue and skeletal growth of the scleractinian coral Stylophora pistillata. Coral Reefs, 22(3), 229–240. https://doi.org/10.1007/s00338-003-0312-7

Tambutté, S. et al. (2011). Coral biomineralization: From the gene to the environment. Journal of Experimental Marine Biology and Ecology, 408(1–2), 58–78. https://doi.org/10.1016/j.jembe.2011.07.026

Kühl, M., Cohen, Y., Dalsgaard, T., Jørgensen, B. B. & Revsbech, N. P. (1995). Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O₂, pH and light. Marine Ecology Progress Series, 117, 159–172.

Hobbyist literature and industry documentation

Fauna Marin (2024). Knowledge Base: Calcium, Magnesium, Potassium, Boron, Bromine, Strontium, Fluor/Fluorid, Iodine, Iron, Manganese, Vanadium, Molybdenum, Copper, Nickel, Cobalt, Barium, Selenium, Rubidium. https://www.faunamarin.de/en/knowledge-base/

Holmes-Farley, R. (2024). “Randy’s Thoughts on Trace Elements.” Reef2Reef. https://www.reef2reef.com/ams/randys-thoughts-on-trace-elements.951/

Holmes-Farley, R. (2024). “Randy’s Elements to Dose.” Reef2Reef. https://www.reef2reef.com/threads/randys-elements-to-dose.1030557/

Aslett, C. G. (2024). Real Reef Talk: Intro Q&A — Nutrients, DOC, Refugiums & Trace Elements. Reef Ranch / reefranch.co.uk.

Ner, S. / Tropic Marin Channel (2024). Trace Elements A⁻ and K⁺ — video transcript. Universität Oldenburg / Tropic Marin.

Books

Borneman, E. H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. Microcosm / TFH Publications.

Delbeek, J. C. & Sprung, J. (1994). The Reef Aquarium, Volume 1. Ricordea Publishing.

Delbeek, J. C. & Sprung, J. (2005). The Reef Aquarium, Volume 3. Ricordea Publishing.