DKH?

It means degrees Karbonat Haerte (degrees of carbonate hardness, translated from Deutsche). It is merely a unit used to describe the concentration of carbonate (and bicarbonate--total carbonate) hardness in the water. While alkalinity is a different measure (I think that article describes that, but I don't remember), it doesn't really matter, since carbonate makes up the VAST majority of alkalinity ions in seawater. Therefore, using it works just as well (one reason, however, to avoid high borate salts, like seachem). True alkalinity (or acid neutralizing capacity) is measured in meq/L, but most kits that measure in milliequavalents per liter still measure carbonate alkalinity.

In a nutshell, it is the water's capacity to resist changes in pH, whether up or down. Seawater is actually able to best resist upward pH changes, though it also resists downward ones, as well. This parameter is important not only for pH stability, but for organisms that utilize these ions, along with mainly calcium to construct shells and skeletons.

Those two articles above are great references!! I have used many of Randy's articles to further my "reef education".

In fact, all of his articles are great!