Alkalinity is one of the most important paramater's of a reef tank. What's strange about alkalinity is it's not as straight forward as something like calcium. Calcium is low, so we add calcium to the tank. That's easy. With alkalinity it's just a bit different. Alkalinity is not really something in the tank, you don't get a bottle of alkalinity and add some to your tank. One definition is that it's a chemical property of the water to resist ph change upon the addition of acid. There are several different chemical's in sea water that contribute to this. Here is the list.
HCO3- (bicarbonate) 89.8 %
CO3-- (carbonate) 6.7 %
B(OH)4- (borate) 2.9 %
SiO(OH)3- (silicate) 0.2 %
MgOH+ (magnesium monohydroxylate) 0.1 %
OH- (hydroxide) 0.1 %
HPO4-- and PO4--- (phosphate) 0.1 %
You'll notice that bicarbonate and carbonate make up the bulk of the chemical species that contribute to alkalinity. These are what we, as reef aquarists, are interested in. These are what coral's use to produce their aragonite skeleton. Actually bicarbonate is taken up by coral's, but carbonate and bicarbonate can interconvert in seawater. They are also used during photosynthesis by both symbiotic algae in corals and micro and macro algae in the tank as a carbon source.
Though there are all these thing's that make up the total alkalinity of the tank, obviously it's the coral building of the bicarbonate and carbonate that we are interested in. And even though resistance to ph drop is important we measure alkalinity to ensure that there is enough bicarbonate and carbonate in the water. but there may be a problem. You'll also notice that borate is third on the list. This is important because unlike bicarbonate and carbonate, borate is not used in calcification, but it may play a significant role in total alkalinity. Remember these number's are for natural sea water. The number's in artificial sea water can vary dramatically. Many salt's have elevated borate level's to begin with, it help's to maintain ph. It's obviously helpfull for fish only tank's where there is no emphasis on calcification, but in our reef tank's we don't really want any more of it than nessicary. Keep an eye on the ingredient's in the alkalinity buffer's you use. For example a popular brand is Kent's superbuffer dkh, this addative contain's a high percentage of borate. Lately they've come out with another addative called coral builder wich is a bicarbonate carbonate mix and is much more suited to our use. When using such salt's and addatives you may end up with elevated borate, and with that comes diminished bicarbonate for a given alkalinity.
Test kit's may read alkalinity in three different units, ppm CaCo3 equivelant's, meq/l and dKH.
50 ppm = 1 meq/l = 2.8 dKH.
Reff tank recommendation's being
125-200 ppm , 2.5-4 meq/L, 7-11 dKH
An important thing to consider is the ammount of alkalinity in the water compared to calcium. During calcification 50 ppm of alkalinity are consumed for every 20 ppm of calcium. So more than twice the ammount of bicarbonate is used compared to calcium. Since recommended reef tank level's of bicarbonate are only 175 ppm alkalinity can drop extremely fast in relation to calcium. For this reason it is extremely important to test for and maintain adequate alkalinity in our tank's.
