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Water Parameter
Unit(s): 1 meq/l

= 50 ppm CaCO3

= 2.8 dKH

Target: 2.5 - 3.5 meq/l
Danger: < 1.5 meq/l
Tendancy: Decrease
Increased: Additive

Water changes

Decreased: Calcification



Testing: Weekly
Maintenence: Sodium carbonate

Calcium hydroxide reactor

Calcium carbonate reactor


Alkalinity is the capacity of the water to resist changes in pH. It is also known as the buffering capacity.

Alkalinity is extremely important for organisms that make skeletons or shells from calcium carbonate, such as corals and snails. It is also used by some algae that contain calcium carbonate such as coralline algae. These organisms need alkalinity for the carbonate portion of calcium carbonate.

Alkalinity is made up of a number of components including bicarbonate, carbonate, borate, silicate, magnesium monohydroxide, hydroxide and phosphate. Of these bicarbonate contributes the most being around 90% of total alkalinity at a pH of 8.0. It is also bicarbonate that is used by calcifying organisms and so this is the component we need to replace with additives such as Kalkwasser.

Carbonate Hardness

In the marine aquarium hobby, the term Carbonate Hardness and the abbreviation KH are used to mean the same thing as alkalinity. This is somewhat misleading for two reasons: Carbonate hardness suggests it is only concerned with the concentration of carbonate/bicarbonate, but alkalinity is more than just carbonate and bicarbonate; Alkalinity has nothing to do with hardness. Many hobbyists kits report KH and use the units dKH (degrees carbonate hardness) but are actually measuring total alkalinity rather than just the carbonate part of alkalinity.


Alkalinity is measured by determining how much acid must be added until the pH of the water reaches a certain point. Fortunately, the available test kits make this determination easy. You simply add a pH indicator dye and then add acid (provided in the kit) until the dye changes colour. The alkalinity is determined by how much acid you added until the colour changed.

Measurement Units

Scientists normally report alkalinity in milliequivalents per litre (meq/L) and this directly indicates how much acid is required. Aquarists use meq/L as well as dKH (degrees carbonate hardness) and ppm (parts per million of calcium carbonate).

The three units can be converted as follows:

Desired Level

Normal seawater has an alkalinity of around 2.5 meq/L. It is wise to maintain alkalinity in the tank of at least this number and up to 3.5 meq/L. Having high alkalinity is of no real benefit but may cause issues with maintaining calcium. See Alkalinty, pH, Calcium & Magnesium. Keeping alkalinity slightly higher than 2.5 meq/L means that fluctuations throughout the day do not result in the alkalinity dropping below 2.5 meq/L.


Alkalinity can be maintained through the use of the following techniques:

Buffering System

Carbonate Ratios versus pH.jpg

Alkalinity consists of the carbonate buffering system, which resists a change in pH. The relative amounts of the components of the buffering system change with pH, see the graph to the right.



How can alkalinity be maintained?

Alkalinity can best be maintained by replacing evaporated water with kalkwasser. If the alkalinity demand is low, only some of the evarporated water needs to be replaced with kalkwasser. If the demand is high, kalkwasser alone may not be enough. A calcium carbonate reactor will also maintain alkalinity and may be able to keep up with demand better than kalkwasser.

See: How to Select a Calcium and Alkalinity Supplementation Scheme

If alkalinity is low, how can it be raised?

You first need to determine if calcium is also low because if it is, you may just need to increase dosing of kalkwasser rather than just adjusting the alkalinity alone. The table below shows approximate "balanced" concentrations of calcium and alkalinity. If your concentrations are close to those in one of the lines in the table, you just need more "kalkwasser" or the calcium carbonate reactor needs to be tuned.

Calcium (mg/L) Alkalinity (meq/L)
440 4.0
430 3.5
420 3.0
410 2.5
400 2.0
390 1.5

If alkalinity is low, but calcium is where it should be or higher, you can use baking soda to raise the alkalinity on its own. Baking soda is sodium bicarbonate and so will raise the alkalinity directly. Baking soda won't have any immediate effect on pH, but will pull it into line over time. One teaspoon of baking soda will raise the alkalinity of 115L of water by 0.5 meq/L.