Water hardness in the marine aquarium is important. The term ‘water hardness’ is maybe too general. It would probably be better to call it carbonate hardness. Another term that is perhaps more recognised in marine hobby circles is alkalinity.
In the sea, the measurement is around 8 dKH (or 2.9 meq/l). This is all starting to sound a bit technical, but it isn’t. All that is really required is for the aquarist to know what the measurement represents, what level should be maintained, and why it is important.
In the sea, there is hardly any change in alkalinity because of the huge volume. In the aquarium it could be a different matter.
Marine aquarium stock carry on with their normal life functions of course. Some of these functions and waste that get into the seawater apply acidic pressure to the seawater. This acidic pressure is constantly trying to lower the pH. (A pH of less than 7 is on the acidic side, 7 is neutral, and above 7 is on the alkaline side.)
As known marine aquariums run from pH 8 to 8.4 for the most part, the ‘perfect’ pH is usually quoted as 8.3. Therefore it can be seen that seawater is alkaline. That’s where it wants to be and where the aquarist wants it to stay. Measuring alkalinity now makes a bit more sense.
Reduction of the pH by acidic pressure is resisted by the alkalinity of the seawater. This is achieved by the carbonate/bicarbonate content. If the aquarist goes to a marine retail shop, there for sale will be additives called alkalinity buffer, pH stabiliser, or something similar. These are carbonates/bicarbonates that are added to the seawater to maintain or increase alkalinity.
The acids that are being added to the seawater are negated by the alkalinity, and the pH is protected against unwanted shifts. However, alkalinity is not constant and can be reduced or even exhausted. As said the seas and oceans are huge, but in an aquarium the acidic pressure could show itself. Routine water changes assist in preventing this. It is considered to be better in a captive system to maintain the alkalinity something higher than natural seawater, and the measurement can be between 9 and 11 dKH (3.2 meq/l to 4 meq/l).
My seawater is maintained at an alkalinity of 3.75 to 4 meq/l. It drifts down during the period between water changes, and is boosted back up (along with the water change). This appears to have assisted in the growth of desirable encrusting algaes and maybe therefore in other life such as snails etc, of which there are many.
Many aquarists do not concern themselves much with alkalinity. This is fine if the livestock is thriving and the pH is as desired and stable.
There are available commercial test kits for alkalinity and they are simple to use. The test kits usually provide a conversion scale to read from, so the aquarist doesn’t have to know what the measurements are scientifically, just what they are of, and the desired level. If an aquarist finds it necessary or just wants to know, this parameter is easily discovered.