Water Chemistry: Calcium

Calcium. “Calcium hardness” is the term used to describe the calcium content of the water, which for natural seawater is about 380 mg/l. Note that this is a simple weight/volume measurement; each liter of seawater contains about 380 mg of calcium. Maintaining this level is a relatively simple matter, as any soluble compound containing calcium can be added to the water to compensate for a deficit. Any compound containing calcium also contains a partner ion, however, and the nature of this partner can have important implications for aquarium chemistry.
Corals, crustaceans, mollusks and coralline algae all extract calcium from seawater, using it to construct their various support structures from calcium carbonate. Early efforts at maintaining the proper chemical environment for these organisms focused too narrowly on calcium alone, largely ignoring the role of alkalinity. This is unfortunate, because the availability of carbonate, the other essential component in skeletal structures, mostly depends upon the pH and alkalinity of the water. In fact, when the alkalinity is high, skeleton-building can still occur, even when calcium is present at a level significantly below that of natural seawater. However, when both alkalinity and calcium concentration are low, corals do not thrive. Conversely, raising the calcium level above about 550 mg/l will result in precipitation of calcium carbonate as chalk, with a concomitant drop in alkalinity, and calcification is made more difficult.

Using limewater for calcium maintenance helps to maintain the pH and alkalinity of the aquarium because the hydroxyl ions from the limewater neutralize some of the acids accumulating in the system. In effect, this prevents the alkalinity from being “used up” and the pH therefore remains more stable. The ideal pH for calcification by living organisms is about 8.40 to 8.45.

Maintaining the proper balance is best accomplished through the use of an automated system for dosing limewater, and an electronic pH meter for monitoring the pH accurately. Adding the limewater by hand, and evaluating pH with a color-change type test kit can be done with more room for error.

Besides aiding in the maintenance of pH, alkalinity, and calcium concentration, the addition of limewater to the aquarium has other benefits. One of these may be the near- total precipitation of phosphate (PO4-3) ions from the water. Undesirable algae growth is often a consequence of excess phosphate. Phosphate is a biolimiting nutrient, meaning that algae cannot grow if starved for this nutrient. Largely due to removal by chemical and biological processes, the water around a coral reef contains virtually no phosphate.

Preparation of Limewater. Caution! Calcium oxide is somewhat caustic. Do not allow the dry powder to contact your skin, and keep it out of reach of children.

For one gallon of limewater, place a rounded teaspoon (1.8 grams, to be exact) of calcium oxide in a clean, clear glass or plastic container that will hold one gallon. Fill the container almost to the top with distilled water. Cap the container and shake well. There should be some undissolved powder on the bottom of the container after it settles; if not add a little more calcium oxide, and shake again.

It is best not to prepare more than a gallon of limewater at any one time, as exposure to the air will cause the calcium to be lost from the solution. The reaction of the calcium ions in the limewater with carbon dioxide in the air produces an insoluble white precipitate of calcium carbonate. This form of lime will need to be cleaned out of the container from time to time, as it will not simply dissolve in water. It should not be added to the aquarium. To remove it, pour a pint of water into the container and add 2 tablespoons of white vinegar. Let stand 30 minutes, and the calcium carbonate deposits should be easy to remove. Rinse well before preparing another batch of limewater.

Summary. Limewater addition benefits the aquarium by:

Adding calcium ions
Increasing alkalinity
Precipitating phosphate
Increasing pH
Facilitating protein skimming.
The ideal pH for calcification is 8.4 – 8.45. The ideal alkalinity is 7-10 dKH. The ideal calcium concentration is about 400 mg/l. From day to day, and even from hour to hour, these numbers will fluctuate, owing to the dynamic chemical and biological processes that are occurring in the aquarium continuously. Since the aquarium, unlike the ocean, is a closed system, the aquarist must intervene to offset the cumulative effects of these changes.