Mike O'Brien
10-16-2006, 12:53 AM
Alkalinity is one of the most important parameters of a reef tank. Alkalinity is a property of water. It's not as straight forward as adding, say, Calcium. Calcium is low, so we add calcium to the tank. That's easy. With alkalinity it's just a bit different. In fact it's a property of water which we measure incidentally. Usually people add a mixture of three different things to raise alkalinity, but there are several different chemicals in sea water that contribute to this.
HCO3- (bicarbonate) 89.8 %
CO3-- (carbonate) 6.7 %
B(OH)4- (borate) 2.9 %
SiO(OH)3- (silicate) 0.2 %
MgOH+ (magnesium mono hydroxylate) 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 corals use to produce their aragonite skeleton. Actually mainly bicarbonate is taken up by corals, but carbonate and bicarbonate can inter convert in seawater depending on pH/CO2. 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 things 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. One definition for alkalinity is that it's a chemical property of the water to resist pH change upon the addition of acid. 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 numbers are for natural sea water. The numbers in artificial sea water can vary dramatically. Many salts have elevated borate levels to begin with, it helps to maintain pH. It's obviously helpful for fish only tanks where there is no emphasis on calcification, but in our reef tanks we don't really want any more of it than necessary. Keep an eye on the ingredients in the alkalinity buffers you use. For example a popular brand is Kent super buffer dKH, this additive contains a high percentage of borate. Lately they've come out with another additive called coral builder which is a bicarbonate carbonate mix and is much more suited to our use. When using such salts and additives you may end up with elevated borate, and with that comes diminished bicarbonate for a given alkalinity.
Alkalinity is closely tied to pH. In fact there is a direct linear rise in pH with a rise in alkalinity. other than alkalinity the only major contributor to the pH of the tank is the amount of CO2 in the water. So basically maintaining adequate alkalinity and keeping good gas exchange to the tank will ensure that the pH is within acceptable range. This is also the reason why one should not try to control the pH with the use of buffers. This is common advice that is frequently given to new aquariusts. Doing this does raise the pH, but in doing so it elevates the alkalinity too high. Low pH is almost always a gas exchange issue or an issue with excess CO2 in the house. So again don't try to control pH with the use of buffers.
Test kits measure alkalinity in three different units, ppm CaCO3 equivalents, meq/l and dKH.
50 ppm = 1 meq/l = 2.8 dKH.
Reef tank recommendations being
125-200 ppm , 2.5-4 meq/L, 7-11 dKH
An important thing to consider is the amount 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 amount of bicarbonate is used compared to calcium. Since recommended reef tank levels 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 tanks.
Happy Reefing.
HCO3- (bicarbonate) 89.8 %
CO3-- (carbonate) 6.7 %
B(OH)4- (borate) 2.9 %
SiO(OH)3- (silicate) 0.2 %
MgOH+ (magnesium mono hydroxylate) 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 corals use to produce their aragonite skeleton. Actually mainly bicarbonate is taken up by corals, but carbonate and bicarbonate can inter convert in seawater depending on pH/CO2. 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 things 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. One definition for alkalinity is that it's a chemical property of the water to resist pH change upon the addition of acid. 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 numbers are for natural sea water. The numbers in artificial sea water can vary dramatically. Many salts have elevated borate levels to begin with, it helps to maintain pH. It's obviously helpful for fish only tanks where there is no emphasis on calcification, but in our reef tanks we don't really want any more of it than necessary. Keep an eye on the ingredients in the alkalinity buffers you use. For example a popular brand is Kent super buffer dKH, this additive contains a high percentage of borate. Lately they've come out with another additive called coral builder which is a bicarbonate carbonate mix and is much more suited to our use. When using such salts and additives you may end up with elevated borate, and with that comes diminished bicarbonate for a given alkalinity.
Alkalinity is closely tied to pH. In fact there is a direct linear rise in pH with a rise in alkalinity. other than alkalinity the only major contributor to the pH of the tank is the amount of CO2 in the water. So basically maintaining adequate alkalinity and keeping good gas exchange to the tank will ensure that the pH is within acceptable range. This is also the reason why one should not try to control the pH with the use of buffers. This is common advice that is frequently given to new aquariusts. Doing this does raise the pH, but in doing so it elevates the alkalinity too high. Low pH is almost always a gas exchange issue or an issue with excess CO2 in the house. So again don't try to control pH with the use of buffers.
Test kits measure alkalinity in three different units, ppm CaCO3 equivalents, meq/l and dKH.
50 ppm = 1 meq/l = 2.8 dKH.
Reef tank recommendations being
125-200 ppm , 2.5-4 meq/L, 7-11 dKH
An important thing to consider is the amount 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 amount of bicarbonate is used compared to calcium. Since recommended reef tank levels 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 tanks.
Happy Reefing.