Updated: Jun 12, 2022
Reef Aquarium Water Chemistry
The complex chemical relationships between the various water parameters we measure in our reef tanks play the primary role in the success of our saltwater aquariums. As each one affects the others we carefully keep track of them to maintain the delicate balance that leads to the beauty and health of our reef tanks.
In this article I will discuss the basic relationships between some of the major water parameters we like to keep track of. Additionally, I will list the recommended ranges for all the major water parameters as a quick reference to at least check to see if you're in the right place.
Let's start with the list of recommended ranges. These ranges are for the reef setup. This type of aquarium includes invertebrates, corals and fish. Fish only, or cold-water marine tanks sometimes have slightly lower temperatures and salinity.
Recommended Aqurium Range
Average Ocean Value
7-12 dkh 2.5-4 meq/L
7 dkh 2.5 meq/L
32-35 ppt 1.023-1.206 specific gravity
34-36 ppt 1.025 - 1.027 sg
74 - 78 degrees Fahrenheit
Variable ~77 degrees Fahrenheit
380 - 450 ppm
1200 - 1400 ppm
0.01 - 0.08 ppm
<0.06 - 2.7 ppm
ORP (oxygen reduction potential)
Calcium, Alkalinity, and Magnesium
Of all the parameters listed above, these three have the closest relationship with one another and each should be closely monitored when changing the other two. All three parameters are heavily tied into the success and growth of your corals, as well as maintaining the natural composition of your saltwater.
Corals take up calcium from the water and use it to create their aragonite (calcium carbonate, CaCO3) skeletons. Calcium can be depleted in smaller aquariums or in setups with many corals before the next water change replenishes calcium levels. We’ll get into the many ways of dosing calcium in another article, but at least make note of the importance of dosing calcium manually or automatically throughout the day or week, in between water changes.
Corals also take up bicarbonate (HCO3-) from the water and convert it to carbonate (CO3--) to build their aragonite (CaCO3) skeletons. Alkalinity is an indirect measurement of the amount of bicarbonate in the water. By measuring your calcium and alkalinity, you have an idea of how much calcium (Ca) and carbonate (CO3--) are available for corals to build their calcium carbonate (CaCO3) skeletons. Bicarbonate is the form within the water column, carbonate is the form corals use after they convert it.
If the bicarbonate concentration is too low (alkalinity too low) coral growth can become severely stunted. Additionally, bicarbonate can be absorbed rapidly. If the alkalinity drops too low, bicarbonates can be depleted quickly and severely crash your system.
If the bicarbonate concentration is too high (alkalinity too high), calcium will precipitate (form solid chunks) on your equipment, rocks, glass, filters, etc). This is not only a hassle to clean, but wastes critical calcium, as the precipitated calcium is no longer biologically available. This will also increase your ratio of alkalinity to calcium, causing an imbalance that can lead to crashing parameters, or at the very least stress upon your coral.
In addition to also being absorbed by healthy, growing corals, magnesium plays a crucial role in the balance of calcium and alkalinity. At recommended levels, magnesium is responsible for preventing excess precipitation of calcium carbonate by attaching to the molecule when it reaches higher concentrations and blocking the addition of more calcium or carbonate. Without magnesium, it would be difficult to maintain natural ratios of calcium and bicarbonate in the water column.
Reef Tank pH
Every reef keeper will give you a different story on the intricacies of pH in a reef aquarium. In this hobby, anything can work for a few people, but we will concentrate on the simple truths that work for most people.
pH is the percent hydrogen or the concentration of hydrogen ions in a solution. Higher concentrations of hydrogen ions means the solution is more acidic. Less hydrogen ions equals a more basic solution. The acidity, or pH, of seawater occurs naturally around 8.0 to 8.3, which is above neutral, or slightly basic. The reef aquarium, depending on your calcium levels and alkalinity measurement, can vary safely from 7.8 to 8.2.
Each aquarium is different. One setup may work perfectly with a pH of 7.8 while another would crash. This is most likely due to the first setup having a higher calcium concentration compared to the other setup.
You should be able to maintain your pH at 7.8 to 8.3 without many issues if you maintain your alkalinity at a minimum of 7 dkh (2.5 meq/L), and your calcium at a minimum of 400 ppm. If your pH raises above 8.2, you may start to see problems with calcification and precipitation of calcium carbonate, causing aesthetical and equipment issues.
As with most organisms, metabolic rate increases when the temperature rises above the optimal thermal comfort range for that creature. Whether the body is tasked with keeping itself warm, or cooling itself down, more energy is expended and therefore metabolism increases. Metabolism is the sum of the normal functions of your body’s cells to maintain normal bodily functions.
Metabolic rate increases as the temperature rises. For this reason, I recommend the lower end of the temperature range for reef aquaria. Artificially increasing the temperature and therefore the metabolic rate of your tank’s inhabitants will increase respiration, which lowers the amount of dissolved oxygen available in the water column.
Aside from increased respiration, higher temperatures also prohibit the solubility of oxygen in water. The higher the temperature, the less oxygen present in the water column. If your power goes out and your aquarium remains still for some time, a lower temperature will help keep oxygen levels higher until the water surface can be disturbed to allow gas exchange.
Nutrients (Ammonia, Nitrite, Nitrate, and Phosphate)
In reef aquariums, macro, micro, and nuisance algae consume nutrients. Fish, coral and motile and sessile invertebrates do not. Due to the open and vast volume of water, nutrient levels in natural ocean water remain very low. If nutrient levels in the aquarium rise above even a 100th of a part per million, only your nuisance algae will benefit. Other inhabitants will become stressed and unhealthy.
Phosphate is a particular nuisance due to its proclivity to rise quickly without proper chemical filtration. If you are having algae issues, test your phosphates first. If they are higher than .01 ppm, look into investing in a reactor filled with granular ferric oxide (GFO). Phosphates above .1 ppm can begin to poison coral directly and should be rectified immediately with water changes and chemical filtration.
Nitrates also build up over time and will contribute to algae growth and fish and coral poisoning. Anything above 5-10 ppm should be addressed with a water change and chemical filtration, while levels above 20-30 ppm should be considered an emergency in the reef aquarium.
Trace Elements (Silica, Boron, Iron, Strontium, Iodine)
There is a long list of trace elements that contribute to buffering capacity and the growth and maintenance of corals in the reef aquarium. Unless you have a specific deficiency in a certain element, most salt mixes or trace element mixes will supply the necessary amounts.
Some aquarists like to supplement iodine in the form of iodide, and strontium as they have been linked to coral coloration. Iodine exists in many forms in ocean water, and trying to control it in the reef tank can easily be more trouble than it’s worth.
In the end, use a good salt mix and a trace element supplement if necessary, but beginners should avoid dosing individual trace elements unless you know you have a deficiency, or you have coral that is known to absorb one element more readily over others.
The most important concept to take away from this article is the proper concentration ranges of the key parameters. The second most important concept is how those parameters interact with one another. The third most important concept is to make changes to these parameters slowly. When you raise your calcium from 400 to 450 ppm, or your magnesium from 1250 to 1350 ppm, make sure it is done slowly. For calcium, 20 ppm a day is normal. Magnesium shouldn’t be raised faster than 10 ppm a day. Alkalinity shouldn’t be raised more than 1 dkh per day. Slow and steady is for the benefit of lowering stress on your fish and coral as parameters change, as well as allowing the chemical reactions between the changing concentrations to occur slowly and deliberately, avoiding a crash.
If you are having a problem with calcium precipitation, pH crashing, or elevated salinity levels, don’t be afraid to experiment with your ratios. Just make sure they are within the recommended ranges, and make changes slowly.
Boodleshire LLC 2021