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In previous articles we have discussed the fundamental water chemistry values that are required for all aquarium and pond owners. These are: ammonia, nitrite, nitrate and pH. This article is going to delve deeper into the important things to test for in a reef aquarium.

Carbonate Hardness (KH)

What is KH?
Carbonate hardness is the measurement of the combined presence of carbonates and bicarbonates in the water. When testing, it is usually expressed as a value in degrees of hardness (dKH). Carbonate hardness provides a buffering capacity to the pH of the water. A correct KH value provides a stabilising effect to the pH.

Carbonates vs bicarbonates:
As KH is measured as the combination of these two compounds, it is important to understand the difference between them. Carbonates (CO32-), bicarbonates (HCO3-), carbonic acid (H2CO3) and carbon dioxide (CO2) all participate in a delicate chain of equilibriums (see illustration), with the relative abundance of each compound being tightly linked to the pH of the water. When the water is acidic (pH <7), the predominant equilibrium is between the dissolved carbon dioxide (also referred to as carbonic acid) and the bicarbonates in the water. Decreasing the pH further, more of the bicarbonates are converted to dissolved carbon dioxide, reducing the KH value. In nature, many acidic environments (parts of the Amazon basin, for example) will measure little to no carbonate hardness in the water. Such water can be described as soft.

When the pH rises above 8.3, the bicarbonate/carbonate becomes the main equilibrium. As saltwater aquariums are recommended to be kept at a pH value of at least 8.0, a balance of HCO3- and CO32- will exist to make up the carbonate hardness. In the same way that the pH of the water can affect the relative abundance between these carbon/oxygen compounds, the opposite is also true; by changing the relative proportions of these compounds, we can also manipulate the pH.

Carbonates, and to a lesser extent, bicarbonates, are the predominant form of carbon that hard corals use to build their calcium carbonate exoskeletons. Corals do have the ability to use bicarbonates but must process the compound in a way to eliminate the extra hydrogen it bears.  Be it through direct utilisation of carbonates or the processing of bicarbonates, as these compounds are used, the relative abundance of these compounds is shifted resulting in the overall process being KH-consuming and pH-lowering.

Why is KH important?
Corals use carbonates (and bicarbonates) to build their exoskeletons. Absorption of carbonates and bicarbonates will reduce carbonate hardness levels. As we have seen, when KH is decreased, pH will also tend to decrease. It is therefore important to monitor the KH weekly to ensure adequate KH values are maintained. To best replicate natural sea water, the KH should be kept at around 7.5 dKH. In some instances, the KH can be higher than this, but it does also require increased monitoring and altering of other parameters, such as calcium and magnesium.

Why is pH test not included in the Reef Multi-Test kit?
Many reef keepers opt to test KH instead of pH. As mentioned, these two values are intrinsically linked. When the KH is around 7.5 dKH, the pH can be relatively safely presumed to be about 8.2 and stable. Most reef keepers therefore focus on testing and ensuring their KH values are correct. It is recommended to check the pH of the reef aquarium periodically to ensure correct values, our Marine Lab pH Test Kit is available as a standalone test kit.

How to increase carbonate hardness:
Over time, natural processes decrease carbonate hardness values and need replenishing. The most common method to restore KH is by using a chemical compound buffer. NT Labs Marine Buffer Powder contains a premium blend of carbonates and bicarbonates that will restore KH and pH to required levels. If levels are critically low, KH buffering should be actioned over a period of days to not create a pH shock - aim for an increase of no more than 1 dKH per day.

Magnesium (Mg2+)

What is magnesium?
Magnesium is a very important element in the reef aquarium and is the second most abundant cation (positively charged ion) in the ocean. Because of its abundance and relatively low utilisation, it’s not usually something that has to be supplemented in a marine aquarium.  There are, however, occasions where magnesium can become reduced, especially when manipulating the calcium concentration in an aquarium. Because of this, it is wise to regularly monitor magnesium concentration. In a reef aquarium, a low magnesium concentration can affect health and growth rate of corals.

Why is magnesium important?
Magnesium serves three main functions in the marine aquarium:
    • Essential for the uptake of calcium for invertebrates to utilise in their exoskeleton formation.
    • Helps to maintain the correct proportions of calcium and bicarbonate/carbonates in the water and thereby helping with pH.
    • Essential for biological systems, including the machinery that drives photosynthesis used by plants and corals (via their zooxanthellae), to energy usage by cells.
The relationship between magnesium and calcium explained:
As highlighted above, magnesium is very important for its relationship with calcium. In natural sea water, Mg2+ has a concentration of 1300-1400 mg/L. This is approximately three times the typical value of Ca2+ found in natural sea water at 420 mg/L. This 3:1 ratio is what is important to keep right in the reef aquarium. It is important to bear in mind that this ratio of 3:1 magnesium:calcium is based on concentration. When considering the proportion by actual number of atoms, due to calcium and magnesium atoms having different masses, there are approximately 5 atoms of magnesium for every atom of calcium. It is also important to remember that is it exceptionally hard to manipulate calcium and carbonate/bicarbonate concentration without having an effect on the magnesium concentration too.

Calcium ions have a natural affinity for carbonate ions. This affinity results in the formation of solid calcium carbonate, removing both usable calcium and pH-buffering carbonates from the water. If left to their own devices, this affinity is so strong it would result in calcium concentration in the tens of mg per litre. Thankfully, magnesium comes to the rescue; it is capable of interfering with this precipitation process, ensuring that valuable calcium and carbonates remain in solution.

What is the ideal magnesium level in the aquarium and how to achieve it:
Magnesium levels should be maintained at natural sea water levels: 1300-1400 mg/l. This will then allow calcium levels to be as high as 450 mg/l. Regular water changes using a quality salt mix will usually maintain adequate magnesium levels in the aquarium. If the Mg2+ levels are testing on the low end, supplementing with a suitable magnesium buffer will restore levels. Many calcium buffers on the market also contain magnesium to maintain the correct ratio of 3:1.

Calcium (Ca2+)

What is calcium?
Calcium is one of the major building blocks of aquatic life. It is the primary element of exoskeletons (invertebrates) and endoskeletons (vertebrates). Corals uptake calcium to form calcium carbonate (CaCO3). Hard coral species (hermatypic) utilise large quantities of  calcium to form a hard calcium carbonate (CaCO3) skeleton. To a lesser extent, soft corals also need some calcium to form sclerites within their softer tissue. An LPS/SPS dominant reef tank will therefore use up significantly more calcium than a soft coral tank. Calcium is the third most abundant positively charged ion (cation) found in sea water, after magnesium and sodium.

Why is calcium important to test?
Keeping the correct levels of calcium in the reef aquarium is important to sustain healthy corals and invertebrates. Calcium is used to form the exoskeletons and forms an important component of hardness in the water. A successful reef aquarium will use available calcium in the water and levels will gradually deplete. Maintaining the correct concentration of calcium is critical. It should be kept at 380-450 mg/l, with natural sea water averaging 420 mg/l. Levels below this range result in poor coral growth. If the Ca2+ levels are too high, calcium binds with carbonates and precipitates, making it unavailable to invertebrates and depleting KH.

How to maintain correct calcium concentrations:
Keeping the correct calcium levels is achieved by regular water changes or calcium additives. In reef aquariums, calcium supplements will be the popular choice as levels can deplete rapidly. It is vital to test for calcium levels before using any product to ensure it is not overdosed.

Remember, it is important to also test for magnesium when considering calcium. The ideal ratio of 3:1 Mg2+ to Ca2+ should be adhered to ensure calcium is kept available and prevented from depleting KH.

A quick word about general hardness (GH) & Brackish Water:
Some fishkeepers who have made the transition from freshwater to saltwater might wonder why there aren’t any general hardness (GH) test kits available for marine aquariums. GH is the total concentration of magnesium and calcium ions in the water. In freshwater habitats, ‘soft’ water is when the GH is low and ‘hard’ water is when the GH is high. There is no such variation in saltwater habitats – with a few exceptions, the makeup of sea water around the planet is remarkably constant. Furthermore, the combined concentration of magnesium and calcium in freshwater, while variable, is on an order of magnitude different compared to sea water.  Converting the numbers into comparable units, the GH of sea water would be in the region of 370 dGH!

A move from freshwater to brackish aquaria also poses difficult conundrums – should freshwater or marine test kits be used? Due to the high concentration of salts and minerals in sea water, a weaker solution of sea water (i.e. brackish), tends to make the properties of the water more akin to sea water than freshwater. For example, the general hardness will be well in excess of the freshwater test kits, and the elevated KH will most likely give a high pH. For these reasons, our Marine Lab Test Kits are the preferred choice.

Phosphate (PO4)

Where does phosphate come from?
Phosphate is an oxygen-containing compound of phosphorus that finds its way into the aquarium through several means. It can be introduced through fish food, fish waste, sea salts, tap water and phosphate-based buffers.

Why test for phosphate?
If left unchecked, elevated phosphate can fuel nuisance algae and inhibit healthy coral growth. Phosphate is found in natural sea water (as phosphorus) at an average concentration of 0.01 mg/L to 0.06 mg/L. For optimal coral growth and minimal nuisance algae, aim for 0.03 mg/L PO4-P.  Contrary to the idea that lower phosphate is always better, and while it tends to be in abundance, some phosphate is essential for life, so aiming for 0 mg/L shouldn’t be the goal. In fish only aquariums, phosphate poses less of a risk as only extremely high concentrations can affect fish health. It will however continue to fuel unsightly algae at a prolific rate.

A quick word about phosphate and its units of measurement:

The concentration of phosphate (PO4) is classically reported in terms of phosphorus as mg/L PO4-P (e.g. ignoring the contribution of mass by the attached oxygen). This is done as there can be multiple sources of phosphate-like compounds, with different ‘weight’ to them. Reporting in terms of phosphorus attempts to use the common denominator of phosphorus and allows for measurements to be compared. Unfortunately, in every-day speech, measurements are reported either as mg/L (without specifying either PO4 or PO4-P), or incorrectly as “phosphate”, when phosphorus is implied.

How does phosphate affect coral?
Studies have shown high PO4 levels can negatively impact healthy coral growth. High phosphate levels encourage a higher proportion of brown algae to grow within the coral tissue. This affects overall colouration of the coral and turns them brown. Excessive phosphate can also restrict calcium carbonate uptake, affecting skeletal formation. Nuisance algae can also affect coral health by outcompeting for light and space.

How to reduce phosphate levels:
To keep phosphate levels low, regular water changes using a reef-grade quality salt, which will be lower in phosphate, will help. Using RO/DI water will ensure no additional PO4 is introduced through tap-water. Growing algae in a refugium provides a natural solution to removing phosphate. For chemical phosphate removal, use NT Labs Phosphate Remover permanently in the filter and replace once exhausted. The filter media is deemed exhausted once phosphate levels begin to rise. Alternatively, instantly remove PO4, dose NT Labs Anti-Phos when levels are tested high.

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