Best substrate to maintain a stable pH

[robert]

What would you consider the best substrate to maintain a stable pH?

 

[robert]

I guess this one is too simple...is the consensus crushed coral (aragonite)?
Anyone have a different pick?

 

[robert]

I'm trolling a bit on this one...
Seven for crushed coral (aragonite) and one for foam...Would it be a surprise if the consensus was wrong?...That aragonite is not only not the best choice but is actually the worst choice...

Any more votes?

 

[Selter904]

I have not voted b/c my vote would be a guess, I am new to the hobby and use aragonite currently, and would have to say I do have difficulties maintaining optimal ph. I am interested to see what others have found the best success with.

 

[robert]

This is one of those cases where the consensus is wrong. It is such a fundamental assumption that aragonite should provide the optimal buffering for a reef system that it has gone largely untested. I chose aragonite as well.

So here's the truth - as experimentally determined in both loaded and unloaded tanks (meaning with and without reef inhabitants)

The tanks with inert substrates - foam and quartz - maintained their pH in the optimal range (7.8-8.4) for upwards of a month without additional buffering being required.
Oyster shell and dolomite were next holding optimal pH for a week or more.
Tanks with Aragonite / crushed coral substrate were the worst. With pH dropping below the optimal range within only a couple ofdays. Dropping to 7.4 within the month.

This does'n mean that aragonite isn't the best substrate - but it does demonstrate that it is not the "buffer" that everyone assumes. You know what they say about the word assume...
Left alone...aragonite will naturally buffer your tank to the 7.4 range - 10x higher concentration of H+ ions than is optimal.

From: Marine Water pH Control.
By: Leo G. Morin, Ph.D

"Contrary to the prevalent assumptions, we have accumulated
extensive data and experience that indicates that marine aquar-
ium pH is better and more easily controlled with inert substrates
such as foam, quartz, coated dolomite, oyster shell, than with
dissolving substrates, most notably, crushed coral.


Ten 40 gallon tanks filtered with crushed coral all exhibited the
same symptom of rapid pH drop from an initial 8.4 - 8.6 to 7.5 -
7.6 within a few days. With no intervention, and no life to sup-
port, three of the tanks ultimately fell to 7.4 after one month and
were stable at that pH thereafter. One tank, with no intervention,
but with a small biological load, behaved similarly. Three other
pairs, with and without a biological load, were treated, as re-
quired, with sodium carbonate, sodium bicarbonate...


Ten other tanks were filtered through dolomite, crushed oyster
shell, quartz, and foam. All of these had a biological load. Both
dolomite and crushed oyster shell showed a minor pH drop to
7.9 - 8.0. after a few days, then dropped gradually to 7.6 over a
30 day period. Carbonates raised the pH and were able to
maintain it for about a week. Marine BufferTM maintained the pH
for close to a month. Compared to the crushed coral tanks, all
other ions were fairly stable...

Best pH stability was secured with
the other inert substrates. (Quartz and foam) Even untreated, these tanks main-
tained a pH 7.9–8.0 for several weeks before intervention with
carbonates...

It is evident from our experience and the understanding of buff-
ers described above that the addition of calcium ions in the form
of carbonates (lime, crushed coral) will ultimately lead to a pK
drop, and ultimately, to a buffer at pH 7.6. This experience and
the consequent conclusions have been confirmed by several
hobbyists and professional aquarists. For the same reasons, the
use of bicarbonate or carbonate to raise pH only increases the
capacity of the buffer and temporarily raises pH, but does not
significantly raise the pK of the buffer. To buffer at pH 8.3, it is
essential to have the proper balance of counterions to control
the pK of the buffer. It is not sufficient to merely increase the
capacity of the buffer...

Further, it should be self-evident that this is more easily main-
tained if the aquarium substrate is inert and does not release ions
into the water that will cause a drop of the pK."

Any thoughts? Are you surprised?

 

[NanoNathan]

Wow I didn't know that. Why does every one use crushed coral then?

Sent from my tank using tapatalk

 

[wysiwyg]

cause it naturally adds calcium and other minerals when disolving in your tank

 

[AZDesertRat]

Aragonite and other medias do not dissolve at the pH we keep our tanks! It takes a low pH in the 6.5 to 6.8 range to dissolve it and our tanks are in the 8.0 to 8.2 range which will not dissolve media. This is one of the big misunderstandings in the hobby.

 

[Young Frankenstein]

I use thick aragonite for a substrate and i disagree with the above foam statement, what does foam offer ? anyway i also don't believe in many research results since they tend to contradict one another, they basically have the answer they want to get to before the research starts. What i believe in is the following photo of my ORP and PH calibrated, steady and up to 8.5+ at times

 

[Young Frankenstein]

Not a drop of foam in my tank never !!!!!!!!!

 

[robert]

Like I said above - I too would have said aragonite. I think it is safe to say that while it doesn't dissolve at the pH of our systems, it has a solubility so it contributes to the ionic balance of the tank. I also think its safe t to say that the pH of our systems varies considerably depending on where we take the measurement. Is the pH the same an inch down in your substrate? In my system, the return water flows up through the sand-bed, so the pH is close to the main body of water, but a foot away next to the sump, the dephosphate/denitrate chamber is 7.5 (or less).

The action of a buffer is to resist a change in pH away from a natural balance point. It does this by either pulling in a H+ ion or liberating a H+ ion depending on which side of the balance point the pH of the solution is. So if you add an acid (net gain of h+), the buffer sucks up H+, if you add a base, the buffer disassociated and frees a h+ ion...always around the point where pH=pK for the buffer. We have many such buffers in our tank. Its the same principle by which ammonium NH4 turns into ammonia NH3 in base environments (releases a H+ ion) and turns back into ammonium in acid (grabs hold of H+ ions). Its a buffer too.

The carbonate system in a tank with an aragonite substrate has a natural buffer balance point around pH7.5. So as your trying to push the pH up, the ions liberated from the aragonite are trying to pull it down,

Frank - your right to question of all the various studies out there - you have to read carefully to see what they are really trying to tell you. Its easy to draw wrong conclusions, but your statement that you have a thick aragonite bed coupled with a picture of your pH controller says volumes. If aragonite kept your pH stable why the expensive monitor? I bet your dosing like crazy to keep your pH up - I am.

I'm old. I kept saltwater tanks in the 70's when quartz substrate was more the rule than the exception. We mostly only did fish, but I don't remember ever really worrying about pH. I'd test once a month or so and it was almost always good. Very rarely did I have to add any buffer to the system. So when I read the article - it resonated with my own experiences.

As one more thing to consider - bicarbonate is one of the primary buffers of the human blood stream - its natural pH balance point - 7.4.

 

[cparka23]

Nice explanation of buffers, robert. I'd only offer one small note for you to consider:

[...] I think it is safe to say that while [aragonite] doesn't dissolve at the pH of our systems, it has a solubility so it contributes to the ionic balance of the tank.

The solubility of calcium compounds is extremely poor. The exceptions are calcium halides (i.e. CaCl2 for aquarists). As the mineral composition of aragonite is calcium carbonate, it's insoluble.

 

[robert]

I won't pretend to understand the Carbonate mineral solubility in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system. But I would have to ask how calcium reactors work if crushed coral is so stable - or how sulphide oxidizing bacteria free calcium from aragonite rock while consuming alkalinity - sulphide reducing bacteria do the same or how organics change the precip points of aragonite over reefs. Isn't it a mistake to overlook biological activity?

How would you otherwise explain the study cited?

Finally - If aragonite is inert in the reef tank - what is its value?

 

[mainereefer]

I have always had a mix of dolomite and aragonite I would recomend this mix to anyone unless you plan on having sand sifting fish the dolomite is hard and sharp

 

[Chameleon]

Like I said above - I too would have said aragonite. I think it is safe to say that while it doesn't dissolve at the pH of our systems, it has a solubility so it contributes to the ionic balance of the tank. I also think its safe t to say that the pH of our systems varies considerably depending on where we take the measurement. Is the pH the same an inch down in your substrate? In my system, the return water flows up through the sand-bed, so the pH is close to the main body of water, but a foot away next to the sump, the dephosphate/denitrate chamber is 7.5 (or less).

The action of a buffer is to resist a change in pH away from a natural balance point. It does this by either pulling in a H+ ion or liberating a H+ ion depending on which side of the balance point the pH of the solution is. So if you add an acid (net gain of h+), the buffer sucks up H+, if you add a base, the buffer disassociated and frees a h+ ion...always around the point where pH=pK for the buffer. We have many such buffers in our tank. Its the same principle by which ammonium NH4 turns into ammonia NH3 in base environments (releases a H+ ion) and turns back into ammonium in acid (grabs hold of H+ ions). Its a buffer too.

The carbonate system in a tank with an aragonite substrate has a natural buffer balance point around pH7.5. So as your trying to push the pH up, the ions liberated from the aragonite are trying to pull it down,

Frank - your right to question of all the various studies out there - you have to read carefully to see what they are really trying to tell you. Its easy to draw wrong conclusions, but your statement that you have a thick aragonite bed coupled with a picture of your pH controller says volumes. If aragonite kept your pH stable why the expensive monitor? I bet your dosing like crazy to keep your pH up - I am.

I'm old. I kept saltwater tanks in the 70's when quartz substrate was more the rule than the exception. We mostly only did fish, but I don't remember ever really worrying about pH. I'd test once a month or so and it was almost always good. Very rarely did I have to add any buffer to the system. So when I read the article - it resonated with my own experiences.

As one more thing to consider - bicarbonate is one of the primary buffers of the human blood stream - its natural pH balance point - 7.4.

Just to play devils advocate, this thread was about best substrate to maintain stable pH ie. less change not pH8.0-8.2. Aragonite is by far the best choice to maintain stable pH as it is the best buffer out of all the substrates mentioned. it will just help maintain a stable pH at 7.4. Starboard and styrofoam have no affect on buffering capacity and therefore they do not draw the pH down, however they do not maintain it at high levels either. This lack of affect could be attributed to the scientific results mentioned. This means that whichever substrate is the best buffer maintains a stable pH the best. BTW the only time I ever measure pH is when I am acclimating inverts...

 

[robert]

Just to play devils advocate, this thread was about best substrate to maintain stable pH ie. less change not pH8.0-8.2. Aragonite is by far the best choice to maintain stable pH as it is the best buffer out of all the substrates mentioned. it will just help maintain a stable pH at 7.4. Starboard and styrofoam have no affect on buffering capacity and therefore they do not draw the pH down, however they do not maintain it at high levels either. This lack of affect could be attributed to the scientific results mentioned. This means that whichever substrate is the best buffer maintains a stable pH the best. BTW the only time I ever measure pH is when I am acclimating inverts...

Ok - fair criticism of the way the poll was worded...The intent was "Which substrate is best used to maintain a stable tank pH at levels seen on a natural reef ~8.2"

 

[cparka23]

I won't pretend to understand the Carbonate mineral solubility in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system. But I would have to ask how calcium reactors work if crushed coral is so stable - or how sulphide oxidizing bacteria free calcium from aragonite rock while consuming alkalinity - sulphide reducing bacteria do the same or how organics change the precip points of aragonite over reefs. Isn't it a mistake to overlook biological activity?

How would you otherwise explain the study cited?

Calcium reactors work because calcium carbonate solubility increases in acidic environments. AZDesertRat shared this with us in his post a few before mine. Essentially, CO2 (gas) + H2O (liquid) = H2CO3 (acid).

For CaCO3 in neutral water, you can get a miniscule amount of calcium to dissolve. Even though some of the carbonate will form bicarbonate, driving the solvation of CaCO3 further, it's still a super-small amount that will be dissolved. Randy Holmes-Farley (hereafter, "RHF") suggests that a little aragonite sand in your freshwater reservoir could have the tiniest of effect this way, but he is quite clear that aragonite will not dissolve in the reef aquarium.

To attempt to explain the article, let me first say this disclaimer: I am an armchair chemist -- I could be wrong on some of these points and will happily take correction. Now, it seems to me that the investigators misattributed their finding to the release of calcium ions released from crushed coral substrate.

It is evident from our experience and the understanding of buffers described above that the addition of calcium ions in the form of carbonates (lime, crushed coral) will ultimately lead to a pK drop, and ultimately, to a buffer at pH 7.6.

Again, straight CaCO3 solubility is a fraction of a percent. RHF wrote an article (link) in Advanced Aquarist that offers an alternate explanation. To paraphrase: aquarium water is supersaturated with calcium and carbonate. When you add a bit of solid calcium carbonate (i.e. aragonite), it causes calcium carbonate to precipitate on the surface (ever grow crystals as a kid?). This crystal growth consumes alkalinity from the water column, reducing buffering capacity. As dolomite is a "mixture" of calcium/magnesium carbonate, it - likewise - is a surface where precipitation can occur. Since the inert substrates are not carbonate based, they do not provide the same surface for carbonate precipitation. As they do not consume the buffer, the authors of the article experienced less need for maintenance. Hence, starboard and foam look like "better" substrates.

Finally - If aragonite is inert in the reef tank - what is its value?

It looks natural, helps in nitrogen fixation, and reflects a bit of light, I suppose. The value of an aragonite sandbed is in the sandbed, not the aragonite. These are mostly just my opinions, though. Sorry for the long post.

 

[Young Frankenstein]

Guys i have this qestion, what would happen if i let my RO sit for a day in a dark bucket with coral skeleton, is it going to desolve some calcium ?

 

[robert]

First - thanks for the reply. I had a set of assumptions which the article I posted seemed to call into question. I'm trying to understand - but I too am a armchair chemist and not a good one.

Calcium reactors work because calcium carbonate solubility increases in acidic environments. AZDesertRat shared this with us in his post a few before mine. Essentially, CO2 (gas) + H2O (liquid) = H2CO3 (acid).

Rhetorical question. I knew the answer. The solubility of calcium carbonate increases 100 fold as the pH is lowered from 8.0 to 7.0.

For CaCO3 in neutral water, you can get a miniscule amount of calcium to dissolve. Even though some of the carbonate will form bicarbonate, driving the solvation of CaCO3 further, it's still a super-small amount that will be dissolved. Randy Holmes-Farley (hereafter, "RHF") suggests that a little aragonite sand in your freshwater reservoir could have the tiniest of effect this way, but he is quite clear that aragonite will not dissolve in the reef aquarium.

by Randy Holmes-Farley » February 11th, 2003, 11:12 am

It is hard to get enough to dissolve that way to support all but minimal demand for calcium and alkalinity, but it clearly does dissolve to some extent in many tanks.
Here is an article by Julian that describes one of the premier folks (Jaubert) that initially got aquarists interested in such systems:

http://www.advancedaquarist.com/issues/ ... eature.htm


"by Randy Holmes-Farley » February 12th, 2003, 11:03 am (http://www.reefs.org/forums/topic25814.html)

Aragonite will not dissolve at any appreciable rate if the pH is not reduced below that of the tank itself, because the water is always supersaturated with respect to calcium carbonate.
If the pH is lowered for any reason inside a sand bed, then the calcium carbonate can begin to dissolve. The pH down inside my sand bed is substantially below that of the water above the sand (mid to upper 7's compared to 8.4 above it).

The pH in the sand can be lower do to either aerobic or anaerobic respiration where organic materials are broken down. Both produce about the same net amounts of acid.
That said, if the sand is aerobic and organics are breaking down, that necessarily means that there is significant O2 exchange between the water column and the sand.
If there is substantial O2 exchange, then there is likely to be a lot of H+ and OH- exchange (and other acids and bases), and that will limit the pH drop that is experienced in the sand.

So in that sense, low O2 regions may be more likely to experience sand dissolution, but that has little to do with the O2 itself.
One problem is that even if the pH drops low enough for some CaCO3 to dissolve, the calcium and carbonate/bicarbonate are going to be slow to get out, for the same reasons that the acids and bases in the water column are slow to get in.

So it is not an especially efficient process for dissolving sand. It is also dependent on the degradation of organics in the sand, which is likely to be dependent on the nature of the organsims living and moving about in the sand.
Randy Holmes-Farley

To attempt to explain the article, let me first say this disclaimer: I am an armchair chemist -- I could be wrong on some of these points and will happily take correction. Now, it seems to me that the investigators misattributed their finding to the release of calcium ions released from crushed coral substrate. Again, straight CaCO3 solubility is a fraction of a percent. RHF wrote an article (link) in Advanced Aquarist that offers an alternate explanation. To paraphrase: aquarium water is supersaturated with calcium and carbonate. When you add a bit of solid calcium carbonate (i.e. aragonite), it causes calcium carbonate to precipitate on the surface (ever grow crystals as a kid?). This crystal growth consumes alkalinity from the water column, reducing buffering capacity. As dolomite is a "mixture" of calcium/magnesium carbonate, it - likewise - is a surface where precipitation can occur. Since the inert substrates are not carbonate based, they do not provide the same surface for carbonate precipitation. As they do not consume the buffer, the authors of the article experienced less need for maintenance. Hence, starboard and foam look like "better" substrates.

This makes quite a bit of sense as aragonite is super-saturatred - further dissolving of calcium carbonate should be precluded. As you state, It's not the contribution of ions (as suggested by the article) but rather the availability of a suitable surface upon which calcium carbonate precipitation can readily rake place - consuming the buffer capacity of the water.

This leads to a question in my mind - calcium carbonate has a pK value somewhere close to 9. Could calcium carbonate preferentially precipitate leading to a relative predominance of bicarbonate with a pK closer to 7.8? (resulting pH less than 7.8)

Next question - precipitation of aragonite from saturated seawater is inhibited by mg bound to the aragonite seed crystal - is this perhaps one mechanism by which mg deficiency influences the alk/calcium balance?
It would have been nice if the author of the article I cited, included the mg data for the tanks in his study.

Last thought - sulphate concentration directly inhibits aragonite precipitation from aragonite saturated waters. Do you know of any downside to elevated sulphate levels in a reef tank?

It looks natural, helps in nitrogen fixation, and reflects a bit of light, I suppose. The value of an aragonite sandbed is in the sandbed, not the aragonite. These are mostly just my opinions, though. Sorry for the long post.

No - thank -you for the long post. It has helped my understanding considerably. In the end - it seems that the contention of the article - that a inert substrate leads to more stable tank parameters as it relates to pH is entirely possible, if not probable. Looks, light and nitrogen fixation are attributes of other substrates as well.

@Frank - yes RODI will dissolve somewhere in the neighborhood of 15mg/liter of the coral skeleton - eventually.

 

[robert]

Ph stable in barebottom tanks? - Reef Central Online Community

"So a substrate such as aragonite sand will not keep my Ph stable?

That is a common misconception.

This article has more on what impacts pH:

Low pH: Causes and Cures
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com
and
High pH: Causes and Cures
High pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com

__________________
Randy Holmes-Farley"