calcium and alkalinity

Reef Tank Chemistry – Calcium and Alkalinity

There are a number of parameters you need to monitor to maintain a healthy and successful reef tank. Even though seawater contains every element in the periodic table (see, among the most critical are Alkalinity and pH, Salinity, Temperature, Calcium, Magnesium, Phosphate, and Ammonia. There is a second tier of parameters less critical but still important to know about and may require monitoring depending on your specific tank type. This article covers Alkalinity, pH, and Calcium together as they influence each other and understanding the relationship will help you maintain a successful reef tank.

Calcium (Ca++)

Maintaining the proper calcium level is imperative to a healthy, growing reef tank . Many corals, mollusks, and algae need calcium to make their skeletons and other structures. They obtain calcium and carbonate from the surrounding seawater and use the process of calcification create calcium carbonate, CaCO3, That then forms skeletons, shells, and other structures. Normal seawater has a calcium level of about 410 ppm (higher in the deep ocean) and most reef aquarists try to keep slightly higher than natural calcium levels. Residence time is the length of time a calcium ion hangs around before precipitating as a solid like calcium carbonate. In the open ocean this time is estimated to be a few million years. In a reef tank it can less than one week.

An important aspect of calcium is that in seawater it is supersaturated and in the right circumstances it will precipitate as solid calcium carbonate. In your reef tank you see this manifested in its deposition on heaters, pump impellers, and any other warm objects. (Calcium carbonate is slightly less soluble as temperature rises). Also important to understand is that calcium levels are tied to pH in that they tend to counteract each other. Increasing calcium forces alkalinity to decrease and vice versa. High demand tanks may need supplemental calcium added through the addition of various products or equipment.

In low and moderate demand tanks, water changes alone are generally sufficient to maintain normal seawater levels, depending on the level your salt mix provides. It does not appear that boosting calcium concentration above natural levels enhances calcification (that is skeletal growth) in most corals. While experiments show that low calcium levels limit calcification, they also indicate that levels above about 360 ppm do not increase calcification.1 Depending on tank size and inhabitants, you will find calcium levels range from 380 to 450. Maintaining normal seawater levels (or to elevate levels in high demand tanks) requires some type of calcium supplementation. There are five basic options for accomplishing this:

  • Kalkwasser (Kalk): This is a dry powdered form of lye that is mixed with water to create a solution high in calcium and has a very high pH. This solution is slowly dripped into the sump, often at night when the pH of the tank tends to be lowest.
  • Liquid Calcium Supplements: There are many liquid calcium products. Using these products requires monitoring alkalinity as well and then uses a buffering product to maintain alkalinity levels in your tank.
  • 2-Part Liquid Supplements: One part adds calcium and trace elements; the other part is a buffer/alkalinity component. 2-Part supplements elevate salinity, so you will need to watch this and make any needed adjustments.
  • Dry Calcium / Alkalinity Products: These are 2 separate parts used in concert to control the calcium level. Both must first be dissolved in water before being dripped into the tank and they must be dripped separately, with a break between
  • Calcium Reactors: This is a cylindrical tube containing aragonite chips that has water pumped through in a recirculating fashion. Carbon dioxide from a compressed air tank is then slowly bubbled through the water and aragonite column to create an acidic condition, dissolving some of the aragonite into the water. A small dribble of water, rich in calcium is released from the reactor and allowed to drain into the sump. Depending on the reactor media used, strontium and magnesium are also maintained at proper levels. The water leaving the reactor has a low pH. Some systems have a second stage on the reactor that is used to dissolve additional minerals using the same amount of CO2. This conserves CO2 consumption which in turn helps raise the pH of the water leaving the reactor so that it has less effect on the pH of the tank. Calcium reactors seem to be very popular on high demand systems.


Alkalinity is the measure of how much acid (H+) is needed to bring the pH to a specific level. There are different types of alkalinity but we’re concerned most with Total Alkalinity (TA). This is the amount of acid need to lower the pH of a sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3–] could be converted to carbonic acid [H2CO3]. For normal seawater, this endpoint is about pH = 4.2 and as a result total alkalinity tests have been invented that determine how much acid is required to lower the pH into the 4-5 range.

pH is a scale used to measure the acidity or alkalinity of water. The scale ranges from 0, most acidic, to 14 which is the most alkaline, and 7 being neutral (pure water). The normal trend for pH in a saltwater tank is more acidic (downward) due to the addition of acids into the aquarium in the form of CO2 from respiration, nitric acid from biologic filtration, and organic acids from metabolic processes. What you want is to maintain a pH around 8.1. Alkalinity test kits have a pH indicating dye (its color changes with change in pH) and an acid to lower the pH. You add acid to the sample until the dyes turn color, which is set to occur in a specific range. Many kits use more than one dye to help make the shift (endpoint of titration) sharper.

Anything that absorbs protons (H+) when the pH is lowered is counted toward alkalinity. In seawater there is a variety of contributors, and in reef tanks the list is even longer. The equation for total alkalinity (TA) in normal seawater is:

TA = [HCO3] + 2[CO3] + [B(OH)4] + [OH] + [Si(OH)3O] + [MgOH+] + [HPO4] + 2[PO4] – [H+]

Of these, bicarbonate (HCO3) and carbonate (CO3) are the two that contribute the most to alkalinity in seawater. In reef tanks, other ions, like phosphate and borate, along with organics like polygluconate, EDTA, and citric acid, can contribute to total alkalinity. This can make it difficult to know how much of the measured alkalinity is due to bicarbonate and carbonate and if it is the amount needed to satisfy the needs of the corals in your tank.

You should now see why alkalinity is important for reef tanks: Corals and other organisms in your tank deposit calcium carbonate in their skeletons and other body parts. They do this by taking it up from the water that they inhabit. Since there is no easy way to measure carbonate or bicarbonate we use fact that these two make up the majority of alkalinity in seawater and use alkalinity as a reasonable measure for the bicarbonate and carbonate in the water that is available to your corals. Normal seawater has an alkalinity of 2.5 meq/L or 125 ppm CaCO3 equivalents or 7dkH. Common alkalinity values for reef tanks are 2.5 – 4meq/L or 125-200 ppm CaCO3 equivalents or 7-11dkh.

Units to measure alkalinity can be confusing and you will see it expressed in Milliequivalents per Liter (meq/L); parts per million (ppm), and degrees of carbonate hardness (dKH)

1. The clearest unit is milliequivalents per L (meq/L): For a 1 millimolar solution of bicarbonate, the alkalinity is 1 meq/L. For a 1 millimolar solution of carbonate the alkalinity is 2 meq/L (because it combines with two protons for each molecule of carbonate).

2. Many kits represent alkalinity in terms of the amount of calcium carbonate that would need to be dissolved in fresh water to give the same alkalinity and is typically reported as ppm (parts per million) calcium carbonate. To convert alkalinity expressed as ppm CaCO3 to meq/L, divide by 50.

3. The German term dKH (degrees of carbonate hardness), or just KH (carbonate hardness) is a confusing term because it has nothing to do with hardness and has been twisted by the marine aquarium hobby to mean the same as total alkalinity.

4. The best means of controlling your tank pH and alkalinity is to reduce causes of pH swings: Remove all uneaten foods and fish waste on the tank on a regular basis. The generally accepted method for stabilizing pH is performing regular partial water changes. This refreshes the natural buffers in water and restores the trace minerals in the aquarium’s water. You can use a simple doser to automatically add buffers, calcium, and other essential trace elements and supplements. Commercial products are available to correct for low or high pH and calcium reactors can provide a no-hassle solution to control radical pH and alkalinity problems in some tanks. (see calcium section)

More Reading –

1 A compartmental approach to the mechanism of calcification in hermatypic corals. Tambutte, E. Allemand, D. Mueller, E. and Jaubert, J. (1996) J. Exp. Biol. 199, 1029-1041.