This is not a complete scientific discussion or description of alkalinity. It is simply intended as a base, as there isn’t a requirement for an aquarist to be a scientist and understand all. A basic understanding allows the aquarist to ensure the seawater in his/her aquarium is at the parameter desired and why.
Right, that said, alkalinity then. Alkalinity of seawater is the ability of the seawater to resist certain change. Alkalinity can also be called Carbonate Hardness. Seawater is on the alkaline side of neutral. To quickly describe this, pH is measured on a scale of 0 to 14, with 7 being neutral. Above 7 is into the alkaline side, below 7 is into the acidic side. Aquarium seawater is, generally, maintained between 8.0 and 8.4. So, as can be seen aquarium seawater is on the alkaline side.[tag-tec]Alkalinity[/tag-tec] resists changes towards the acidic side of the pH scale. Another way of putting this is that it ‘buffers’ the seawater – it has an acid binding capacity. The life cycles etc within the aquarium are continuously trying to push the seawater towards the acidic side, and this push is cancelled out by the buffer capacity of the seawater. This buffering is enabled by carbonates and bicarbonates.
The [tag-tec]alkalinity of water[/tag-tec] is measured in either dKH, meq/L, or mg/l. (If it is wished to convert dKH to mg/l, simply multiply dKH by 17.9.) Whichever scale you choose does not really matter. They give the same answer with different numbers, in the same way that distance can be measured in miles or kilometres. Test kits are readily available and will give instructions, and come with a table so the reading can be easily obtained. It is important to follow the instructions carefully.
Natural seawater has a measurement of around 8 dKH. (143.2 mg/l). The sea has an enormous volume, and in the aquarium it is better to maintain a reading of between 9 and 11 dKH (161.1 and 196.9 mg/l). It should not be necessary to raise the seawater alkalinity higher than 11 dKH.
Having measured the seawater, if it is decided to increase the alkalinity level (for example, because there is a low pH reading), this can be achieved by adding carbonates and bicarbonates (adding ‘bases’, this is the opposite of adding acids). These are available commercially, and are easily applied. The products are often described as KH and/or pH Boosters or Buffers. Do not attempt to raise the alkalinity too rapidly, follow the manufacturer’s instructions for the net gallonage of the aquarium. Changes in the marine aquarium should be gradual.
The maintenance of a good alkalinity level has great benefits. It not only protects from drops in pH, it can help keep corals, particularly hard corals, in good health and assist the growth of colourful encrusting algae.
Routine water changes, proper seawater circulation to allow gas exchange etc, and routine measurement of parameters including pH and alkalinity will help maintain a beautiful display aquarium. Once the aquarium has settled and matured, the aquarist may feel the water testing regime can be relaxed. This is fine, although occasional testing should still take place. Any sign of deterioration, overall water testing should be initiated.
Those who might wish to read a little more should follow this link, which is interesting and straightforward:
It is also recommended that along with alkalinity, the aquarist should have a look at the meaning and control of pH.