Kalkwasser only for Alkalinity

[AllSignsPointToFish]

So, I met a local reefer on Friday that says he has had an SPS-dominant reef for the last 10 years or so. We got to talking about pH when I asked him how he maintains a pH of around 8.4 (since I have problems due to high ambient CO2 levels in my house). The long story made short is that he does not dose any bicarbonate/carbonate like I do, and he said he only uses calcium hydroxide (kalkwasser) in his top off water.

Is this even possible? I would have thought that corals would need supplemental dissolved carbonate to build their skeletons?

If this is possible, where does that carbonate come from? Is it from ambient absorption of CO2, slow dissolution of aragonite sand/rock, or some other source? It seems to me his 400-gallon SPS tank would require a LOT more carbonate than either of these two sources could provide alone.

He also said he runs a calcium reactor which confused me further because, although I've never run a calcium reactor, I was under the impression that the reactor operates by dissolving calcium carbonate with CO2, thereby liberating both the carbonate and calcium ions for the corals to use. However, a downside of this approach is that this process lowers pH, so how can this guy keep his pH at 8.4?

Anyway, I'm skeptical by nature, but I wanted to understand before I dismissed this guy's approach outright or chalked up the apparent discrepancy to instrument error.

Thoughts?

 

[Randy Holmes-Farley Verified Community Expert]

Sure, it is possible. I used nothing but limewater for 20 years. The carbonate comes by combination of the hydroxide in the limewater with CO2 from the air. That is also why it boosts pH: it depletes CO2.

But an SPS heavy tank will often need more than can be added with limewater due to limitations on evaporation and hence how much can be added each day. His use of the reactor demonstrates this point, as it is adding both calcium and carbonate.

 

[-Logzor]

This is the old-school way of reefkeeping. Kalk also adds a small amount of calcium if I remember correctly.

Some reading material:

http://reefkeeping.com/issues/2005-01/rhf/index.php

 

[Willz]

Kalkwasser provides both calcium and alkalinity. He is using this to maintain his pH. More than likely the calcium reactor takes care of the bulk of his calcium and alkalinity needs.

 

[AllSignsPointToFish]

The more I think about this, maybe I understand what is happening.

Seeing as how the calcium reactor provides a balanced solution of calcium and carbonate, is it possible that he just adds enough kalwa

Sure, it is possible. I used nothing but limewater for 20 years. The carbonate comes by combination of the hydroxide in the limewater with CO2 from the air. That is also why it boosts pH: it depletes CO2.

But an SPS heavy tank will often need more than can be added with limewater due to limitations on evaporation and hence how much can be added each day. His use of the reactor demonstrates this point, as it is adding both calcium and carbonate.

Yeah, he mentioned that his tank will deplete about 300 ppm of calcium per day without supplemental calcium, so his calcium bioload has to be pretty large.

When the CO2 combines with the hydroxide, I would assume it precipitates out in the form of calcium carbonate, right? Should be the same as the precipitate at the bottom of the top off container, so with all of the CO2 sequestered as a solid precipitate in the top off container, how does that help the reef tank? Seems like the only source of available carbonate is from the calcium reactor unless not all of the carbonate in the top off container precipitates.

Maybe I'm just overthinking this thing!

 

[AllSignsPointToFish]

He did mention that his calcium runs 450ppm or higher which leads me to believe the "extra" calcium is coming from the limewater. The alkalinity part has me a bit confused, however.

 

[Elementalj]

The more I think about this, maybe I understand what is happening.

Seeing as how the calcium reactor provides a balanced solution of calcium and carbonate, is it possible that he just adds enough kalwa

Yeah, he mentioned that his tank will deplete about 300 ppm of calcium per day without supplemental calcium, so his calcium bioload has to be pretty large.

When the CO2 combines with the hydroxide, I would assume it precipitates out in the form of calcium carbonate, right? Should be the same as the precipitate at the bottom of the top off container, so with all of the CO2 sequestered as a solid precipitate in the top off container, how does that help the reef tank? Seems like the only source of available carbonate is from the calcium reactor unless not all of the carbonate in the top off container precipitates.

Maybe I'm just overthinking this thing!

I wouldn't feel bad, I've been doing this for almost 30 years and I still don't fully understand everything. I just know that it works and is as reliable as sunrise. [emoji41][emoji106]

 

[rock_lobster]

carbonate is dissolved in the topoff water in saturated solution it goes into the tank. This depends on the amount of evaporation and topoff a tank can handle to determine whether kalk will meet the demands. My sps tank has relatively low flow high stock and only evaporates about 2 gallons a week. Not even nearly possible to keep up with carbonate demand at that rate.

 

[MaiReef]

http://reefkeeping.com/issues/2006-06/rhf/index.php

This explains how both calcium, alkalinity and pH rise with kalkwasser.
I actually am adding a stirrer now that my needs are greater than a bi-weekly water change

 

[mcarroll]

so with all of the CO2 sequestered as a solid precipitate in the top off container, how does that help the reef tank?

What?

If that's how it goes, you're doing it wrong.

Look up Randy's "still reservoir" system....not everyone has that much space, but it illustrates the right way to go about it. (No mixing.)

 

[AllSignsPointToFish]

So, to summarize, would it be fair to say:
1. The calcium reactor provides a balanced mixture of calcium and carbonate but reduces pH, and,
2. The limewater adds additional calcium but contains enough hydroxide ions to absorb excess CO2 from the calcium reactor, thereby raising pH?

I'm trying to decide if this approach is something I'd like to try, so apologies for the dumb questions

 

[AllSignsPointToFish]

What?

If that's how it goes, you're doing it wrong.

Look up Randy's "still reservoir" system....not everyone has that much space, but it illustrates the right way to go about it. (No mixing.)

You mean you don't get a white residue at the bottom of your top off container like I do? The kalkwasser directions indicate this is normal, although mine might be excessive due to elevated household CO2 levels.

 

[mcarroll]

Some can be normal*, but if you mix it more than once, you're really doing it wrong.

*I don't think it's pure, for one thing; you might be adding too much material, for another.

 

[AllSignsPointToFish]

Some can be normal*, but if you mix it more than once, you're really doing it wrong.

*I don't think it's pure, for one thing; you might be adding too much material, for another.

I only mix it once to dissolve it, then add it to the container. I mix 1 teaspoon of dry powder per gallon of DI water.

 

[Randy Holmes-Farley Verified Community Expert]

This is the old-school way of reefkeeping. Kalk also adds a small amount of calcium if I remember correctly.
p

Yes, it adds slightly more than enough calcium for the amount of alkalinity added.

 

[Randy Holmes-Farley Verified Community Expert]

You mean you don't get a white residue at the bottom of your top off container like I do? The kalkwasser directions indicate this is normal, although mine might be excessive due to elevated household CO2 levels.

Yes, a crust and ppt on the bottom is normal.

 

[Randy Holmes-Farley Verified Community Expert]

So, to summarize, would it be fair to say:
1. The calcium reactor provides a balanced mixture of calcium and carbonate but reduces pH, and,
2. The limewater adds additional calcium but contains enough hydroxide ions to absorb excess CO2 from the calcium reactor, thereby raising pH?

I'm trying to decide if this approach is something I'd like to try, so apologies for the dumb questions

It doesn't add "additional" hydroxide. It adds all of its alk as hydroxide. And yes it adds essentially balanced calcium and alkalinity.

 

[AllSignsPointToFish]

It doesn't add "additional" hydroxide. It adds all of its alk as hydroxide. And yes it adds essentially balanced calcium and alkalinity.

What is the residence time of the hydroxide once added to the tank? Is it essentially converted to carbonate alkalinity immediately or does the amount of available CO2 control the rate of conversion of the hydroxide ions to carbonate alkalinity?

I had always assumed that the hydroxide ions remain in solution after addition to the tank and only served to raise the pH, but now it seems like the actual mechanism is that the hydroxide reacts with CO2 to form a carbonate/bicarbonate buffer. Is that what is going on?

 

[dragon99]

http://reefkeeping.com/issues/2005-01/rhf/index.php

The calcium ions in the solution obviously supply calcium to the aquarium, and the hydroxide ions supply alkalinity. Hydroxide itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef aquarium, it quickly combines with atmospheric and dissolved carbon dioxide and bicarbonate to form bicarbonate and carbonate:

4. OH- + CO2 > HCO3-

5. OH- + HCO3- > CO3-- + H2O

 

[Randy Holmes-Farley Verified Community Expert]

What is the residence time of the hydroxide once added to the tank? Is it essentially converted to carbonate alkalinity immediately or does the amount of available CO2 control the rate of conversion of the hydroxide ions to carbonate alkalinity?

I had always assumed that the hydroxide ions remain in solution after addition to the tank and only served to raise the pH, but now it seems like the actual mechanism is that the hydroxide reacts with CO2 to form a carbonate/bicarbonate buffer. Is that what is going on?

Hydroxide reacts into the carbonate/bicarbonate/CO2 system as soon as it is added. The reaction itself is measured in very tiny fractions of a second.

But it takes time to mix throughout the tank and even longer for CO2 to be pulled in from the air (which is why the pH drops over a period of hours after a bolus dose of hydroxide).


Here's a blurb from one of my articles:

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide (OH–) itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank carbon dioxide (CO2) and bicarbonate (HCO3–) to form bicarbonate and carbonate (CO3—):

OH– + CO2 → HCO3–

OH– + HCO3– → CO3— + H2O

Once in the aquarium at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 dKH). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to an aquarium unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition, and that is acceptable for additions that would add significantly less than 0.5 dKH of alkalinity to the tank, but an addition of 1.4 dKH (0.5 meq/L; the equivalent of adding 1.2% of the tank volume in saturated limewater or 14 grams of solid calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up by about 0.6 pH units from where ever it started).

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).