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Article: [CA] Alkalinity - How Corals Grow

  1. [CA] Alkalinity - How Corals Grow

    1 Comments by Shadowramy Published on 09-04-2009 05:26 PM
    Alkalinity - How Corals Grow (Part 1)
    Shadowramy, 2009

    For me one of the hardest concepts to grasp in reef keeping was that of alkalinity. Most of us reef keepers come from a background where the focus is on pH, alkalinity was always just a simple little test that gave results of good or bad.

    There are many excellent articles on alkalinity today by some major scientists and chemist therefore will “tread lightly” in my beginner friendly approach about this topic. At the end of this article I have gathered several reference articles that one should really look over for further information.

    As the reef hobby (or should I call it lifestyle) has grown we are starting to see shift favoring alkalinity opposed to pH in the reef aquarium. With the advent of ULNS, two-part dosing and balling systems we are finding that alkalinity has a lot to do with a successful reef as well as help us in troubleshooting issue such as poor growth, RTN and coral survival. Gone are the days of a simple calcium reactors that we set and forget, today we really need to pay careful attention to our alkalinity and avoid its negative impacts if we are to be successful.

    We are now seeing dominate SPS systems were alkalinity is becoming the primary “go-to” for testing and troubleshooting, but many reef keepers are having a difficult time understanding what alkalinity is and its relationship is to calcium, pH and magnesium.

    It is my hopes that in this article, I am able to provide some useful information about the chemical make up of alkalinity and its significance in our reef systems today.

    What is Alkalinity
    For along time I believed that alkalinity was no more than a buffer against pH swings, although this is true I think a further explanation is required, therefore let’s starts out with “What is Alkalinity”?

    Alkalinity is (1) a measurement of bicarbonate, specifically, how much bicarbonate is being used (2) secondly, alkalinity acts as a pH buffer which helps or hinders pH stability, the lower the alkalinity the more unstable the pH, the higher the alkalinity the more stable but the more both living and non-living precipitation (fallout) you are going have. Alkalinity in natural seawater (NSW) is around 7 dKH.

    I think most people think that alkalinity is a physical thing, like baking soda, calcium or another additive or supplement. Strangely enough, alkalinity is nothing more than a measurement, like one inch or five pounds. The technical answer for alkalinity is that it measures the amount of acid required to reduce the pH to about 4.2 or how much acid it takes to lower the pH to the point where bicarbonate is converted to carbonic acid (Holmes-Farley, 2002). Wow, that’s sure is a mouthful, but what we really want to know is why is that significant to us?

    Bicarbonate Is What We Seek
    As stated, alkalinity is a measurement, it measures a point where bicarbonate converts to carbonic acid, lets just say that Alkalinity in a sense, answers the question of what our bicarbonate level is. So why is our bicarbonate level important? Corals need to extract bicarbonate (as well as calcium) into their systems and convert these it into calcium carbonate in order to help them form their skeletons and grow. Therefore, like a calcium test tells us how much calcium is in our system, alkalinity tells us when bicarbonate can be converted to carbonic acid.

    So what we really need is a bicarbonate test kit, right? Well bicarbonate test kits are very difficult to produce so instead we use a workaround, alkalinity, the measurement of when bicarbonate is converted to carbonic acid.

    This is how an alkalinity test kit works; by dropping little drops of acid (titration) into a measured test vial of saltwater; the drops are then counted until the pH of the water reaches 4.2 (the point where bicarbonate is converted to carbonic acid) and the color changes, the counted drops amount typically equals your alkalinity. As a note, alkalinity can be measured differently depending on the test kit you have, dKH or meg/l, or even ppm with the most typical being dKH.

    So then, why is it import to know why bicarbonate is converted into carbonic acid if corals take in bicarbonate? To get that answer we first need to take a look at calcium.

    How Do Corals Use Calcium and Alkalinity – a.k.a. How Calcification Works
    Calcium is much easier to understand but very still complex in nature, corals use calcium in combination with carbonate to grow and form their skeletons (calcium carbonate). Calcium levels in natural seawater (NSW) are between 380-400ppm.

    Let me state this in very simple terms and then if you would like a more detailed description of the process take look at, “Chemistry and the Aquarium by Holmes-Farley”.

    Think of a typical coral as lollypop, you have a wrapper (skin or tissue), an outer shell (termed ECF) and then the hard core (the skeleton). Calcification takes place in-between the tissue layer (wrapper) and the hard core (center). For calcification to work, both calcium and carbonate must get to the center of the lollypop to help form the skeleton.

    For calcium it is pretty direct, from the water column through the tissue and into the shell. Of course there are several chemical and biological processes along the way, but this is basically the path.

    For carbonate it is a much less direct path but basically, details aside, bicarbonate enters the water column and is changed at either the tissue level or the shell level into carbonate.

    Once both calcium and carbonate are in the shell, they are combined and form aragonite which is a form of calcium carbonate and can be used by corals to grow. At what levels of calcium and carbonate are best for certain corals is still undetermined, however, this is were the original idea of why alkalinity and calcium levels should be evaluated past NSW, i.e., the more calcium carbonate the more growth, therefore, elevate your calcium and alkalinity past NSW levels to increase growth.

    To recap the process; calcium and alkalinity (in the form of bicarbonate) are in the water column of the aquarium, bicarbonate and calcium enter the coral, bicarbonate is converted to carbonate and the carbonate calcium forms into aragonite (which is a form of calcium carbonate) and the coral precipitates (extracts, takes in) the calcium carbonate and grows.

    How to Make Your Own Coral Skeleton
    Here is a simple experiment to help visualize the process of calcification and how corals precipitate calcium carbonate into their skeletons:
    Take 1 cup of vinegar, one stick of broken up blackboard chalk (calcium), a cup of tap water and six teaspoons of baking soda (bicarbonate).

    1. Mix 1 cup of vinegar and the broken stick of chalk in a glass. Let it sit for a couple hours. Now take the clear liquid and pour it into another cup for later and discard any film that was left on the surface. (Calcium)

    2. Do the same for the 1 cup of tap water, mix with 6 teaspoons of baking soda, stir it then let it settle. Then pour out the clear liquid in another glass discarding any film that was left on the surface. (Carbonate)

    3. Combine the calcium (vinegar and chalk mixture) with the carbonate (baking soda and water mixture) into one glass.
    What is going to happen is that a white precipitate is going to form and then settle. This represents a coral extracting calcium carbonate from the water and forming a skeleton.

    Also in each step a saturation point was reached with either the calcium or carbonate in the glass. In step one where the vinegar and chalk was combined, a certain amount of calcium floated at the top, this is called supersaturation, the vinegar was at the point where it could not hold any more calcium in it and the rest floated to the top, the same was seen with the bicarbonate.

    Coral growth depends on several requirements; temperature, calcium carbonate saturation, salinity, pH, nutrients, flow, etc. The level of each affects process of calcification either in a negative or positive way. Corals grow by combining calcium (ions) with carbonate (ions). The amount of calcium ions is much higher than the amount carbonate ions in seawater. Therefore, calcification is controlled by the saturation point of carbonate ions in the seawater.

    Saturation is a hard concept to get at first, but just think of it this way; the higher the level of calcium carbonate the more likely is that it will precipitate. Think worst case, by adding way too much kalk (limewater) you get what looks like a snowstorm in your tank, this is when calcium carbonate has reached past the point of saturation and is falling out of the water (precipitating), the water can not hold anymore because it has been supersaturated resulting in the negative effects of pH rises and alkalinity swings. This happens in our system when calcium, bicarbonate or both have reached a point where the water can not hold any more, the excess is seen settling on our heaters and sumps (heat attacks this) in the form of hard white deposits.

    Under saturation would be the opposite of supersaturation, when there is not enough calcium carbonate for corals to precipitate, resulting in low calcium levels and slow growth. Corals can take the calcium carbonate in and use it, but just at a much slower pace, which means slower growth.
    To summarize, calcium ions and bicarbonate ions are freely in the water column. When a calcium ion and carbonate ion settle on a calcium carbonate surface they stick together to form calcium carbonate and they become part of the solid, and thus have been depleted from the water column.

    We also know that seawater is already supersaturated with calcium carbonate and if the water contains an excess amount of calcium and carbonate then precipitation of calcium carbonate can begin and continue until a saturation point is reached. If calcium and carbonate are not in excess or below the saturation point then very little precipitation can exisit.

    Calcification Inhibitors
    So you basically have a reserve of calcium in your system and a reserve of bicarbonate in your system. As each ion “hooks up” witch each other they form calcium carbonate. But sometimes there are inhibitors that can stop this process; basically a calcium ion cheats on the carbonate ion with either phosphate or magnesium. Therefore, the calcium’s “cheating” behavior stop corals from precipitating calcium carbonate into their systems. Make sense?

    Okay, so Mr. Calcium ion is floating around, he settles on a surface, along some Ms. Phosphate ion, they end up sticking together; therefore Ms. Bicarbonate has to find a different Mr. Calcium to form calcium carbonate with. You can image the implication of this if there are massive amounts of phosphate in the system, very little available calcium carbonate for corals to use. Magnesium works in the same way, but there is a benefit we will talk about later.

    Ionic Balance
    As calcium carbonate is being taken up by the corals it is also being depleted from the water column. Therefore, it must be replaced in some manner. We can measure the amount of depletion by simply taking yesterdays calcium and alkalinity results and subtracting them from today’s calcium and alkalinity results.

    So can we just add a cup of calcium and a cup of baking soda? Not exactly, in natural seawater it gets kind of tricking because calcium and alkalinity compete for space, therefore if you have too much of one it will limit what you can have of the other. You can see this resulting in either a low alkalinity and high calcium, or a high alkalinity and low calcium, what we want to aim for is balance.

    In seawater, there is a much bigger calcium reserve than there is alkalinity (bicarbonate) and if we where to add these in equal amounts we would effectively be unbalance our system. Therefore we must balanced these not by adding the exact amounts of each but rather by adding calcium and alkalinity in amounts in which they could theoretically be combined to form calcium carbonate (what we want, what coral can use). We can do this by knowing the formula for the ionic balance;

    10 ppm of Calcium = 1.4 dKH (.5 meg/l) of alkalinity additive (bicarbonate).

    So then for every 10 ppm of calcium you add, you should theoretically add 1.4 dKH of alkalinity (bicarbonate). This also hold true for the depletion rate, for every 20 ppm of calcium that is depleted, ~ 2.8 dkH of alkalinity will be depleted.

    There is a natural balance here, if you calcium is 500 and alkalinity is 7 you can easily figure you are unbalance and poised for problems.

    Natural Balance

    When Calcium (Ca) is at XXX, then Alkalinity (alk) should equal XXX

    Ca= 390, alk= 4.2
    Ca= 400, alk= 5.6
    Ca= 410, alk= 7 (NSW), pH 8.2
    Ca=420, alk= 8.4
    Ca=430, alk= 9.8
    Ca=440, alk= 11.2
    Ca=450, alk= 12.6
    Ca=460, alk= 14

    So then if I am getting values of 460 ppm of calcium and 7 dKH what does that mean? Well either one of two things is happening, (1) your test is bad, or (2) that calcium is at an unbalanced level and in danger or precipitating out, at those high levels your pH is going to swing and without another binder such as magnesium your alkalinity will plummet further (because alk is still being consumed at 2.8 dkH per 20 ppm of calcium). Follow the progression:

    Day 1 – Ca = 460 , alk = 7
    Day 2 – Ca = 440 , alk = 4.2
    Day 3 – Ca = 420, alk = 1.4

    Let’s say we just add some alk buffer to bring the levels to 11 dKH but don’t add calcium (1.4 dKH – 11 dkH = 9.6 worth of buffer)

    Day 4 – Ca = 400, alk = 11
    Day 5 – Ca = 380, alk = 8.2
    Day 6 – Ca = 360, alk = 5.4

    See how the depletion of calcium is minimal but alkalinity is huge? Your troubleshooting might seem to be all over the place, especially if you are only looking at pH.

    This one good example of why alkalinity is starting to be a primary testing tool, if you know your alkalinity you will know where you should be at in terms of calcium and thus calcium carbonate, but by only looking at the pH swing and adding in a buffer (bicarbonate) you are not ironically balancing your additives and are going to have issues.
    If you are having trouble getting your alkalinity and calcium values in line you should check out Randy's article on fixing alk/calcium problems it gives some really nice graphs that are helpful [See references at the end of this article].

    The Role pH
    Let me briefly talk about pH in the aquarium. I think we all have a basic understanding of what pH is but the role it plays with calcium carbonate is probably different than what you think.

    In natural seawater the calcium level is 420, alkalinity is 7 and pH is 8.2. The pH greatly effects how soluble calcium carbonate is (how many free ions can be in the water before precipitation can happen). At a higher pH level there is lower solubility and respectively, at lower pH there is a higher solubility (more can be held in the water). Why is this important? Because at a higher pH level it is more likely that calcium and carbonate are going to precipitate (fallout). This is shown by the calcium carbonate build up on heaters and pumps and why lime reactors seem to clog up (high pH less soluble). A pH level of 8.4 has twice the supersaturation than that of a pH level of 8.1 and four times that at pH level of 7.8.

    Two Common Examples of Solubility

    A kalk reactor; where the pH is very high thus has a lower solubility, therefore calcium and carbonate (which is supplied by the reactor) are more likely to precipitate into calcium carbonate, faster and cause the “crude” buildup on pumps, heaters and on the tubing of the reactor.

    A calcium reactor; where the pH is very low (6.5) because of CO2, the water is able to hold more calcium and carbonate ions (more solubility) before if precipitates.

    This is a big reason as to why reefs that run higher pH levels have harder time keeping calcium and alkalinity levels stable and are continually “scrapping” off calcium deposits from their heaters and pumps, higher pH is more soluble and more likely to precipitate into calcium carbonate faster. While those with lower pH levels find it rather easy to maintain these levels because there is technically more “room” for the calcium and carbonate ions before the precipitate (hopefully on a calcium carbonate living coral skeleton).

    So does that mean a lower pH level is better? Not exactly, corals and other living organisms require certain pH levels in order to calcify property and demand more calcium and alkalinity that a rock bottom pH could provide, remember NSW has a pH of 8.2.

    The Role of Magnesium
    Magnesium is a little tricky to understand and I must admit I do not fully understand all the impacts that it has on a reef system. However I do know that some amounts of magnesium is taken up (incorporated) by corals, but this is a relatively small amount. The primary benefit of magnesium is to allow carbonate and calcium levels to remain higher and thus more stable.

    Magnesium helps to keep calcium and carbonate at supersaturation levels by holding onto a free carbonate ion. When calcium carbonate becomes supersaturated and starts to precipitate, magnesium can bond to calcium so calcium carbonate doesn’t fall out in to a “snow storm”, or on heaters, pumps, etc (also call abiotic precipitation non-living precipitation). This effectively stops precipitation much like phosphate does but has the additional benefit of being precipitated by coral into their calcium carbonate skeleton.

    Having good levels of magnesium greatly helps with overall stability of calcium, carbonate and pH which makes keeping calcium and alkalinity levels supersaturated without all the abiotic precipitation. Therefore, if you ever have had a time where you are having trouble maintaining calcium and alkalinity levels, magnesium maybe deficient in your system, especially at high pH levels.

    Some even suggest that magnesium helps coral color; I believe this is a myth or maybe a mirage of stable alkalinity and calcium levels.
    Last edited by Shadowramy; 09-08-2009 at 12:05 PM.

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  3. #2

    [CA] Alkalinity - How Corals Grow (Part 2)

    Several years ago the original school of thought was the higher the alkalinity and calcium levels, the faster corals would grow? Scientifically this is seems to be true, it has even been tested on porties species corals and seems to work. It has been the standard advice given to many new reefers for years, alkalinity levels of 10-12 dKH.

    Over the last few years many SPS reefers are finding that the high end of the dKH scale is not always better and that keeping alkalinity closer to NSW of 7-8 dKH has actually led to less issues of abiotic build up, excessive algae issues, RTN and even better growth results. From my personal experiences (7-8 dKH), I have found this also to be true. And during my research on this article I can now understand why my stability is better and why abiotic precipitation is much less.

    Before we get into what is a good alkalinity level, let’s take natural seawater levels (NSW) for reference:
    Alkalinity – 7 dKH
    Ca – 420 ppm
    Salinity – 34-36ppt
    pH – 8.0-8.3
    Magnesium – 1280
    Pre 2006
    Remember that the primarily idea pre 2006 was that raising alkalinity above NSW will increase calcification rates. Thus levels of a 10-12 dKH were recommended as a standard.

    Typical Pre 2006 Reef System
    Ca – 450-500 ppm
    Alkalinity – 10-12 dKH
    Salinity – 33 ppt
    PH – 8.0 – 8.4
    Magnesium – ????
    Sometime ago I did a study on the alkalinity changes amounts reef keepers over the past two years, although my data was very limited (using only Reef Central Tank of the Months) I still think the review was worth a look.

    While looking over several of the Reef Central Tank of the Months you notice a shift in thinking when it comes to alkalinity and calcium levels. Here are some of the results;

    • 60% of the Reef Central Tank of the Months have an alkalinity level between 7-9 dKH.
    • The most common level was between 8-9 dKH (25%)
    • The average level of all the TOTMs was between 8.5 and 9.8 dKH (8.7 and 9.6 with a median score of 9.0)
    There seems to be a new trend of alkalinity levels getting closer to NSW. Over the first 15 months (2006-2007) alkalinity ranged from 8.7 - 9.8 dKH. During the last 6 months alkalinity dropped to ranges of 8.5-9.5. If I threw out the one guy running a 10 dKH the average would be a solid 8 dKH. Six months later I review this again with the new batch of TOTMS and again this trend continued with the last 7 tanks being, 8.5-9.5, 8-9, 9, 9, 9, 7, 8-9.

    In the last 6 months alkalinity is now at 8.4-8.8 with the median being 8.8-9.0. Almost 80% of the TOTM are NOT running a dKH higher than 9. In the last year only 1 person ran a dKH over 10.)
    • 20 of the 25 tanks had a dkH lower than 10.
    • Over 50% of the tanks have 9 dKH or lower. Again, looking over the last six month, now 16 out of 28 run lower than 10 dKH, 5 out of 28 run a dKH of 10 or over, and 7 run somewhere between 8-11 dKH. That is 57% run under a 9, 18% 10 or over and 25% run somewhere between 8-11 dkH.
    Variances – Stability is the Key
    Only 3 of the 25 tanks had a variance of more than 3 points - meaning the swing range is very low and alkalinity is very consistent and stable. Before reef keepers would say, “Somewhere between 8-11”. Now successful reefers know exactly where they are in terms of alkalinity, no more big swing listing of 7-11 they are exact numbers.

    The largest swing listed was at 10 to 12 (3 points). In the last 2 years, no tank had an alkalinity variance greater than 3 dKH, again stable Alkalinity is key.

    Todays Shift in Thinking Parameters
    Calcium; 420 - 450 ppm
    Alkalinity – 7-9 dKH
    Salinity – 36 ppt
    PH – 7.9 – 8.1
    Magnesium – 1280-1300
    We know the importance of what role alkalinity plays as a measurement of bicarbonate and how it does help stabilize pH. No more are general ranges of alkalinity, the more precise and stable the better the results.

    There are many conclusion and ideas we can gain from the RC TOTM analysis alone. Closer to NSW alkalinity levels seems to be the trend, resulting is the appropriate calcium carbonate levels with less issues of algae, RTN, abiotic precipitations and with just as good or better looking corals and growth.

    If you think about it, that should not surprise you. Natural seawater has an alkalinity of 7 dKH and corals grow very well. The increased alkalinity is only suggested as a way to make corals grow bigger, faster not the “gospel” measure that one can not go below. But this increased chance at increased growth comes at a price, more abiotic precipitation and less stable levels which leads to other issues.

    I really believe that the closer to NSW levels of alkalinity is much easier to maintain, and thus resembles better success.


    Reef Aquarium Water Parameters

    Solving Calcium and Alkalinity Problems

    Reef Chemical Calculator

    Do-It-Yourself Magnesium Supplements

    The Chemical and Biochemical Mechanisms of Calcification

    Making Coral Skeletons

    A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH

    When Do Calcium and Alkalinity Demand Not Exactly Balance?

    Chemistry and the Aquarium: What is Alkalinity?
    Last edited by Shadowramy; 09-08-2009 at 12:06 PM.

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