Let's talk about Alkalinity, Calcium and Magnesium in an SPS Tank

[cwegescheide]

<a href=showthread.php?s=&postid=11469478#post11469478 target=_blank>Originally posted</a> by MCsaxmaster
No, with lower alkalinity the pH will be lower, all other things being equal. If pH here really is lower when the alkalinity is higher, then CO2 must covary with alkalinity in this case. Hence, the CO2 becomes significantly higher as alkalinity climbs higher, causing the reduced pH.

cj

Since my supplementation is provided by a calcium reactor I guess I buy that. My thinking was/is the amount of alkalinity supplied by my reactor is buffering my ph, not allowing it to rise above a certain point. EVERY time my ph goes above a certain point (just recalibrated my probe - it was reading 8.6 but I know its not that high) I know my Alk is low. Now I'm not sure if its because the amount of bicarbonate in suspension is low or its the lack of Co2 in the tank.. Since CJ is working on his MS in Marine Biology I will take his word for it

 

[gary faulkner]

<a href=showthread.php?s=&postid=11469478#post11469478 target=_blank>Originally posted</a> by MCsaxmaster
No, with lower alkalinity the pH will be lower, all other things being equal. If pH here really is lower when the alkalinity is higher, then CO2 must covary with alkalinity in this case. Hence, the CO2 becomes significantly higher as alkalinity climbs higher, causing the reduced pH.

cj

Sorry, I read that wrong. If my Ph falls I always check to see if the alk is lower. It has a direct relationship not inverse?

Thanks

 

[Aquarist007]

<a href=showthread.php?s=&postid=11473453#post11473453 target=_blank>Originally posted</a> by gary faulkner
Sorry, I read that wrong. If my Ph falls I always check to see if the alk is lower. It has a direct relationship not inverse?

Thanks

that would be a direct relationship

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=11473453#post11473453 target=_blank>Originally posted</a> by gary faulkner
Sorry, I read that wrong. If my Ph falls I always check to see if the alk is lower. It has a direct relationship not inverse?

Thanks

pH is positively correlated with total alkalinity and negatively correlated with total CO2. Raise total alkalinity, everything else equal, and you'll raise pH. Raise total CO2, everything else equal, and you'll lower pH. Raise both total alkalinity and total CO2 proportionally and you'll maintain the same pH.

These relationships ARE NOT linear, but understanding the general form of the relationships is very useful.

 

[morphlingman]

Great post people. Lots of advice.
One year ago a purchased a two chamber reactor (B-Ionic could not keep up with Calcium demands), filled with Arm media, it worked great for the first 6-8 months, but the past 4 months or so it has difficult. I find myself doing tests and adjustments every two or three days. Over the past 4 months results have been as follows. Alk jumps around 9-16, Calcium also up and down 340-410 and just tested Mg for the first time yesterday 1100 (starting to supplement).
Has anyone had problems with effluent flow going down daily. As an example I could set it at 30 ml/min and the next day sometimes sooner it has gone down to 28, even 26, ml/min. (Yes I have replaced the in & out tubes and cleaned (rinsed) adjustment valves.

 

[melev]

I would check to see if the feed pump to the reactor (from the sump) is unobstructed, as well as the physical connection where it goes into the Calcium Reactor. Mine clogs up with detritus and it is a pain to clean out.

To help keep your calcium reactor's output more consistent and avoid Alkalinity swings like you've posted, I would (and do) use a pH Controller. That way it will turn the CO2 on and off as necessary and your tank remains safer.

 

[MIKE NY]

x-2....I keep a PH probe in the effluent cup so I'll know right away if something is wrong......

 

[Reefun]

I’ve read this thread from the first page and didn’t see anything like my problem. Hope it’s ok to ask this here. I’ve never had a tank with a lot of sps and I plan on adding a lot more. So I’m doing my best to better understand the demands of a sps only tank. I’ve been in the hobby for a long time but this is a little different than my previous setups.
My ph is 8
Mg is 1380
Cal is 440
DKH is now 9-10 but just got it there
In about 100gal
My ph, Mg and calcium were and are stable but my alk would not stay up. I was adding about a teaspoon a day of Kent super buffer -dkh just to keep dkh from slipping below 8. After about two weeks of this I got a test kit for MG and it was and is at 1380. I don’t know how it got there because my freshly mixed salt is at 1100. I switched about a week ago to Kent coral builder and after using about 4oz my DKH is now around 9-10. I’ve ordered a bulk two part kit that should be in next week so I will not continue to use the Kent products at that point.
So basically my two questions are:
1. How could my dkh keep going down while calcium remained relatively unchanged?
2. How did my magnesium level get to where it is now? Nothing was ever added to the tank other than the Kent products mentioned and Kent turbo calcium in very small amounts.
Tank is just over two months old and only in the last month had anything in it that would use calcium

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=12405563#post12405563 target=_blank>Originally posted</a> by Reefun
So basically my two questions are:
1. How could my dkh keep going down while calcium remained relatively unchanged?

The most likely answer, if the result were “real” (see what I mean below) would be the addition of an acid. An acid will reduce alkalinity without necessarily affecting calcium.

In order to measure a change in concentration of something in sea water (e.g., calcium concentration, magnesium concentration, total alkalinity) we have observe a change in concentration larger than the error of our analytical method.

For example, the accuracy with which we can measure calcium concentration in sea water is about +/- 5%, which really isn’t too bad for a hobbyist grade test kit. Since the calcium concentration in NSW is ~412 ppm we can measure the concentration accurately to within about +/- 20 ppm. In other words, if the calcium concentration changes by less than ~20 ppm, we cannot accurately or confidently measure a change in concentration. This also means that if we measure a concentration of 400 ppm, the real concentration should be within roughly the range of 380-420 ppm, but we cannot determine where it is in that range without using a more accurate analytical method.

For total alkalinity we see the same sorts of concerns. We can measure alk. to within a margin of error, but we can’t determine where in that margin of error the real value isâ€"we can just shrink the error by using better methodology. For alk. measurements we can typically see an error of about +/- 1 dKH (or about +/- 0.35 meq/l). So, if we measure 9 dKH, the real value is likely somewhere in the range of 8-10 dKH. If the alk. changes more than 1 dKH we can measure a change in total alkalinity, but we cannot accurately measure a change if the change is not at least that large.

When calcium is removed from the water to build skeletons, two equivalents of alkalinity are also removed:

Ca2+ + 2HCO3- = CaCO3 + CO2

This reaction proceeds as a molar ratio, not as a mass ratio. We normally measure calcium based on mass (ppm calcium is equivalent to grams of calcium per kilogram of SW). If you do the calculation you find that for every 1 meq/l = 2.8 dKH of total alkalinity you remove from the water you also remove 20 ppm calcium.

Recall, we have to remove a MINIMUM of 20 ppm calcium just to be able to accurately measure any change at all. In order to achieve this change in calcium we have removed roughly 3x the amount of alkalinity we need to in order to see a change in concentration.

Imagine, for example, that you measure a drop in alk. of 1 dKH in your aquarium after a couple of days. That is a big enough change to say with confidence that the alk. has fallen by about 1 dKH. How much should the calcium have changed? The calculation is fairly simple: 20/2.8 ~ 7 ppm. That change in calcium is too small to measure, meaning that we would get about the same measured value for calcium. This could give us the impression that alkalinity is falling while calcium is remaining stable, however, this is an artifact that results simply due to the nature of measuring these parameters in SW. There is almost 5 times as much calcium in NSW as there is alkalinity. We can easily measure a 10% change in calcium concentration or total alkalinity in your sea water, but a 10% reduction of alkalinity causes less than a 2% change in calcium due to the difference in background concentration.

The most likely scenario is that both are decreasing, but due to the nature analytical chemistry and statistics, you can only measure that change that is occurring in both by measuring alkalinity.

<a href=showthread.php?s=&postid=12405563#post12405563 target=_blank>Originally posted</a> by Reefun
2. How did my magnesium level get to where it is now? Nothing was ever added to the tank other than the Kent products mentioned and Kent turbo calcium in very small amounts.
Tank is just over two months old and only in the last month had anything in it that would use calcium

No idea…more info please.

Chris

 

[Reefun]

Ok that answers my DKH question. I did add a small amount of calcium so It is very possible it moved less than my test could detect .
But the magnesium is still confusing me. I like the level it’s at but I want to know how it got there so I don’t raise it even higher. I ran the test for mg three times with a Red Sea test. The only thing I’ve read that suggest were the mg could have come from is the label of the Kent coral builder. It reads “Coral builder is designed to have only minimal impact on calcium, MAGNESIUM or strontium levels unlike other buffers.” So could it have come from the Kent super buffer-dkh?
There is really not much else to say about the tank I’ve not added any thing other than the mentioned products. If there is anything more you need to know please let me know.

 

[MCsaxmaster]

Hmmm, do you now or have you in the past used tapwater in the tank (as opposed to RO/DI)?

 

[Reefun]

Never used tap water all water came from my RO/DI and it has only made about 500gal. All filters are good.

 

[Reefun]

Could it have come from a bag of salt that was off?

 

[Aquarist007]

I am still trying to check out the coral builder as to what exactly is in it. I hate it when a company won't list the ingredients of a product.
The statement "minimal impact" on magnesium--that brings up a red flag for me
I'll ask on the reef chemistry forum

 

[Aquarist007]

here is the link to the thread in the chem forum

http://www.reefcentral.com/forums/showthread.php?s=&threadid=1378557

 

[Reefun]

Thanks

 

[matan_grin]

nice post..
learnd a lot from it

 

[khoivo1]

i have the same problem ,,,now i am setting up calcium reactor,,,
how do i increase alk alone,,without raise cal ?? with geo cal reactor,,,
my alk going down very fast,,,but cal is going down slow,,,any one own calcium reactor please help me thanks

<a href=showthread.php?s=&postid=12405563#post12405563 target=_blank>Originally posted</a> by Reefun
I’ve read this thread from the first page and didn’t see anything like my problem. Hope it’s ok to ask this here. I’ve never had a tank with a lot of sps and I plan on adding a lot more. So I’m doing my best to better understand the demands of a sps only tank. I’ve been in the hobby for a long time but this is a little different than my previous setups.
My ph is 8
Mg is 1380
Cal is 440
DKH is now 9-10 but just got it there
In about 100gal
My ph, Mg and calcium were and are stable but my alk would not stay up. I was adding about a teaspoon a day of Kent super buffer -dkh just to keep dkh from slipping below 8. After about two weeks of this I got a test kit for MG and it was and is at 1380. I don’t know how it got there because my freshly mixed salt is at 1100. I switched about a week ago to Kent coral builder and after using about 4oz my DKH is now around 9-10. I’ve ordered a bulk two part kit that should be in next week so I will not continue to use the Kent products at that point.
So basically my two questions are:
1. How could my dkh keep going down while calcium remained relatively unchanged?
2. How did my magnesium level get to where it is now? Nothing was ever added to the tank other than the Kent products mentioned and Kent turbo calcium in very small amounts.
Tank is just over two months old and only in the last month had anything in it that would use calcium

 

[MCsaxmaster]

<a href=showthread.php?s=&postid=12539950#post12539950 target=_blank>Originally posted</a> by khoivo1
i have the same problem ,,,now i am setting up calcium reactor,,,
how do i increase alk alone,,without raise cal ?? with geo cal reactor,,,
my alk going down very fast,,,but cal is going down slow,,,any one own calcium reactor please help me thanks

Calcium will always appear to get used slower than alkalinity. This is an apparent result stemming from the fact that there is ~5x as much calcium in sea water as alkalinity and to produce calcium carbonate it takes 1 equivalent of calcium but 2 equivalents of carbonate alkalinity.

A calcium reactor will return calcium and alkalinity in very close to the same ratio that they are used (1:2) since you are simply dissolving CaCO3 (though in reality there is some MgCO3, a small amount of SrCO3 and several impurities layed down by calcifiers in the tank and present in the reactor media). Since there is a large background concentration of calcium in the water, it takes a heck of a lot of calcification to notice a change in concentration. On the other hand, since there is a far smaller background concentration of carbonate alkalinity it doesn't take as much calcification to be able to notice the alkalinity dropping. The important thing to understand is that we have the *appearance* of different rates of change not because calcium and alkalinity are being sucked out at an imbalanced rate, but simply because of an artifact of measurment.

Thus, probably all you need to do is adjust your reactor a bit higher. That will raise both calcium and alkalinity properly. However, since there's already a lot of calcium, it will be a proportionally small change. Since there is much less alkalinity, it will be a proportionally large change.

In terms of the actual stoichiometry: if you dissolve 1 milliequivalent of CaCO3 in a liter of water you'll raise the alkalinity 1 meq/l and you'll raise the calcium ~20 ppm.

Chris

 

[Aquarist007]

IMO it is not said enough-- MCsaxmaster--thank you for all the detailed posts you make, I and I am sure others have learned alot from you.