This is a measure of the waters buffering capacity or its ability to withstand a drop in PH. (see PH) Ideally this should be in balance with the Ca level (see Magnesium) at about 2.9meq/l or 8.0Dkh (carbonate hardness) with a Ca reading of 420-430ppm.
Corals and coralline algae also effect/deplete Alkalinity as well as Calcium.
In heavily stocked tanks it may be necessary to use a Ca reactor or Kalkwasser drip over night, in lower demand aquariums a balanced 2 part additive or water changes will usually suffice in keeping levels up.
Alkalinity buffers or 'sodium bicarbonate' should only be used as a temporary fix though, as the whole Ca, Alk, Mg relation will need looking at if your having problems.
Remember that Red sea levels are slightly different to NSW (Natural Sea Water) levels where our corals are commonly found. Andy again states that a low salinity reading may make your tests read falsely.
Many hard corals suffer at DKH levels below 7.8 where basal stripping and loss of pigment may be encountered.
