CaCO3 or NaHCO3 to raise kH? - The Planted Tank Forum
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post #1 of 14 (permalink) Old 11-24-2015, 07:49 AM Thread Starter
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CaCO3 or NaHCO3 to raise kH?

Really confused and have found mixed reports all over the web on which one to use. Can someone lay this out to me in a clear, simple explanation? I am using RO Right to raise GH, but am confused on which one to use to raise kH. Would a 50/50 mixture work best? I am really just looking to have a bit of bufferng capacity to keep the pH from swinging too much. I am injecting CO2, but am not injecting A LOT of CO2. I like to keep it on the lower end, just enough to keep rotala. Thanks.
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post #2 of 14 (permalink) Old 11-24-2015, 07:58 AM
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CaCO3 has its benifits, but being able to dissolve easily in water is not one of them.

NaHCO3 will be significantly easier if you're unsure. You may also want to consider KHCO3 or K2CO3 if you rather add potassium when raising KH, then sodium.

Feel free to edit.

Last edited by Audionut; 11-24-2015 at 08:43 AM. Reason: easily
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post #3 of 14 (permalink) Old 11-24-2015, 09:01 AM Thread Starter
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Thanks for the reply. Obviously CaCO3 while suspended will cloud the water. When suspended will it register on a TDS meter or only when dissolved. From your experience how long with the cloudiness last before the ions undergo complete dissolution. Does it take hours/days/weeks?
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post #4 of 14 (permalink) Old 11-24-2015, 09:17 AM
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A TDS meter measures Total Dissolved Solids, so it will only register CaCO3 (or any other salt) when it's dissolved in water, it won't measure the undissolved solids in the water.

The rate of dissolution depends on pH and flow. The lower the pH (more H+), the faster it will dissolve. As pH becomes alkaline, it may not dissolve at all. As water flow creates erosion, so does it increase dissolution, but the water must still have a sufficient supply of H+ (acid). It is the H+ that strips the Ca element from the ion, and attaches to the CO3 ion to form HCO3 and H2CO3.

CaCO3 is not really useful for increasing KH by some amount over some short period of time. There are far better ions for that, including NaHCO3, KHCO3, K2CO3. If you don't want the Na or K addition to the water, I would still recommended their use to raise KH to the wanted level initially. From then, you can dissolve CaCO3 over time and use water changes to remove Na and K while maintaining KH with CaCO3.

Otherwise, if you are prepared to prep the water for a day or two before using it in your tank, then CaCO3 can be used. But this will require some further steps.

If you can provide some further details for your target water parameters I may be able to help further. Else, search for posts by @Diana, they contain useful information regarding GH and KH (and pH).

Feel free to edit.
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post #5 of 14 (permalink) Old 11-24-2015, 09:59 AM
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Quote:
Originally Posted by Corey A View Post
how long with the cloudiness last before the ions undergo complete dissolution. Does it take hours/days/weeks?
Forever! In 100L (25G) of water you can fully dissolve only 2 grams of CaCO3.

Solubility of CaCO3 (20C = 77F): 0,002 g/100mL = 0,02 g/L = 2 g per 100L (25G) of water
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post #6 of 14 (permalink) Old 11-24-2015, 10:43 AM
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Something happens to the Ca element and CO3 ion with the addition of H+.

Ca++ can exist as free cations in a solution, and H+ can bond with CO3--

So yes, the solubility of CaCO3 is very poor in pure water, but can be improved upon.

Last edited by Audionut; 11-24-2015 at 12:15 PM. Reason: add minus
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post #7 of 14 (permalink) Old 11-24-2015, 10:36 PM
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I've been using http://smile.amazon.com/gp/product/B0064GZPU4?psc=1&redirect=true&ref_=od_aui_detailpages00 to raise KH of RO water. It dissolves easily.

I use Seachem Equilibrium for GH.
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post #8 of 14 (permalink) Old 11-27-2015, 04:28 AM Thread Starter
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Thanks for the responses. You guys have been much helpful. For the time being I will dose NaHCO3 and Kent's RO Right until I exhaust the RO Right. It seems that Seachem Equilibrium is a much better product knowing there is no sodium chloride in it. I think I will dose NaHCO3 until I exhaust RO Right, then I can place an order for some potassium bicarbonate and Seachem Equilibrium.

The plan of action is to dose NaHCO3 until 50 ppm, then adding RO right until I achieve 90-100 TDS. It probably will result in more Na ions present than what I desire, but it should work. Does anyone know what concentration of Na ions in ppm is desirable in a planted tank?
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post #9 of 14 (permalink) Old 11-27-2015, 01:15 PM
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Na isn't directly useful to aquatic plants as far as I know. I know that some plants can use Na in place of K in some situations. I don't think 50 ppm of Na will cause a giant issue especially if you are keeping K levels up there. Na is significantly less reactive than K in water.

Apparently Na can also be used by plants in trace amounts to further help with osmotic balance between the inside and out of a plant cell, but I'm pretty sure that is K's chief job.

Edit: I'm currently using NaHCO3 to raise the KH in my new AS tank because the AS literally strips all the KH from my tap (1.25 KH) in less than 12 hours. I've ordered KHCO3 from a brewing company to use instead. I'll use the KHCO3 for my k fertilizer until KH stabilizes. Without adding anything I was running into a situation with less than 0.25 KH and pH with CO2 of 5.4. My bacteria were not multiplying and I had nitrite issues. Since I've been keeping KH at 2 and oxygenated the water at night, the bacteria have come back. Hah and this was with 2 fully cycled canister filters.
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post #10 of 14 (permalink) Old 11-27-2015, 02:15 PM
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That's my understanding also @Positron.

The problem with using NaHCO3 where the substrate is consuming CO3, is that the substrate doesn't also consume the Na, so you are left with adding more NaHCO3, and thus significantly increasing Na to maintain CO3. As you rightly point out, somewhat large concentrations of Na likely isn't a dire affair, but just because something doesn't immediately kill everything in the tank, doesn't mean there aren't better, or just different ways of doing things.

The low pH, and thus high H+ concentration, has a useful role in speeding the dissolution of CaCO3 in the water, and increasing the maximum solubility of CaCO3 in the water. I can't explain the process without chemistry, and I don't understand the chemistry well enough to explain it simply. But I can assure you that H+ and CO3 have a higher affinity then Ca and CO3, and thus the H+ concentration in low pH water will break the CaCO3 bond, leaving free Ca++ cations in the water, and forming HCO3 (and H2CO3). This is useful, since Ca++ is likely far more useful in an aquarium then Na, and if nothing else, allows the reduction of other Ca++ sources such as CaSO4, that simply adds unnecessary levels of SO4 to the water.

Don't take my word for it, throw some shell grit, crushed coral, cuttlebone or other CaCO3 source in your filter and observe.

Feel free to edit.

Last edited by Audionut; 11-27-2015 at 02:54 PM. Reason: add H2CO3
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post #11 of 14 (permalink) Old 11-28-2015, 11:02 AM
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I'm genuinely interested how much my GH an KH will raise by putting...say...15 grams of argonite (CaCO3) in my filter. I've always been skeptical on it's effects particularly because my pH is so low. Putting HCl on CaCO3 is a rather stunning reaction. And you are right...as the pH lowers, the ability for an aqueous solution to dissolve carbonate compounds increases...drastically. I believe pH also effects the dissolution of other pairs like sulfate and phosphate.

Maybe i'll experiment with putting crushed coral (argonite) in the filter's mechanical bay and observing. It beats adding so much SO4 to the aquarium. My current SO4 sources:

CaSO4
MgSO4
K2SO4

I think i'll be able to take out K2SO4 by using KHCO3 to keep my kh atleast 2.
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post #12 of 14 (permalink) Old 11-28-2015, 11:47 AM
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0.68 grams in 10 gallons will raise GH and KH by one German degree. This equals 7.14 ppm of Ca++.

Of course, that is with the entire 0.68 grams dissolved, which will take time dependent on the amount of CaCO3 source added to the water, and the pH. The slow rate of dissolution of CaCO3 is probably it's biggest advantage, since you can add a large source of CaCO3 to the water which will dissolve over time, and thus, maintain good stability. Rather then HCO3 sources which will deplete rapidly.

If your supply of CO2 is consistent, pH is a good indicator of the dissolution rate of your CaCO3 source. If pH rises, you have to much CaCO3 being dissolved, and KH is also rising. If pH drops, you have insufficient CaCO3 being dissolved, and KH is also dropping. However, you should also bear in mind the diurnal effects on CO2 concentration with a constant CO2 supply. Don't even think about using this trick with a pH controller controlling CO2, but then, with varying KH levels you shouldn't be allowing a pH controller to determine CO2 concentration anyway.

Feel free to edit.
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post #13 of 14 (permalink) Old 11-28-2015, 12:21 PM
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Originally Posted by Audionut View Post
0.68 grams in 10 gallons will raise GH and KH by one German degree. This equals 7.14 ppm of Ca++.

Of course, that is with the entire 0.68 grams dissolved, which will take time dependent on the amount of CaCO3 source added to the water, and the pH. The slow rate of dissolution of CaCO3 is probably it's biggest advantage, since you can add a large source of CaCO3 to the water which will dissolve over time, and thus, maintain good stability. Rather then HCO3 sources which will deplete rapidly.

If your supply of CO2 is consistent, pH is a good indicator of the dissolution rate of your CaCO3 source. If pH rises, you have to much CaCO3 being dissolved, and KH is also rising. If pH drops, you have insufficient CaCO3 being dissolved, and KH is also dropping. However, you should also bear in mind the diurnal effects on CO2 concentration with a constant CO2 supply. Don't even think about using this trick with a pH controller controlling CO2, but then, with varying KH levels you shouldn't be allowing a pH controller to determine CO2 concentration anyway.
Right! So I stopped using my pH probe to control CO2 a few years ago. It took me awhile to realize that my actual CO2 levels in the aquarium would fluctuate independent from my pH. Doh! changing KH values again.

Yeah. I've done the math as to how much CaCO3 would raise my KH per gram dissolved, but it's going to be dissolving slowly, and at an unpredictable rate; for all the reasons you listed.

I'm still going to try it. I'm interested how CaCO3 (from argonite) dissolves at pH 5.7, and high flow

15g would be a good starting amount. I'll wiegh it again in 3 days to see how much is left. Happy I bought that ultra accurate and precise electronic scale!
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post #14 of 14 (permalink) Old 11-29-2015, 03:11 AM
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Originally Posted by Positron View Post
but it's going to be dissolving slowly, and at an unpredictable rate;
Sure, I don't have the maths to say it will dissolve this fast, at this pH, but it's not entirely unpredictable.

We know the factors that control the rate at which it dissolves. If you add 15 grams of CaCO3 to your filter, it will begin to dissolve at some constant rate. It is this constant rate of dissolution that allows us to predict it's affect. If you do not change the factors that control the rate of dissolution, then simply, if pH drops, KH is also dropping, add more CaCO3, and if pH rises, KH is also rising, remove some CaCO3. Pretty simple really.

It is only when we change the factors that control the rate of dissolution that we have to consider their effect.

If we consider some substrate that contains H+ (such as from NH4), then we can logically assume that the rate of H+ addition to the water will be greatest when water first contacts the substrate, and that over time as the supply of H+ decreases, the rate of H+ addition to the water will also decrease. In other words, we need some amount of CaCO3 to cover off the H+ addition from the substrate, but over time, the amount of CaCO3 needed to cover off this H+ will decline as the source of H+ declines.

But, and here is where the advantage of CaCO3 really shines, as the rate of free H+ ions to the water is reduced, so is the dissolution rate of CaCO3 reduced. As the rate of free H+ ions to water is increased, so is the dissolution rate of CaCO3 increased. So there is harmony between the chemical species. Balance. It is only when you increase or decrease the rate of free H+ ions or CaCO3 supply to the water by some significant amount that you throw the balance out of whack. In other words, the reaction is predictable, until you change the predictability.

As free H+ and CO3 bond in the water, at low pH the end chemical species will be H2CO3 (a weak acid). However, H2CO3 concentration in the water will not continue to build exponentially, since the final transformation state of H2CO3 will control the concentration of H2CO3 in the water, CO2 + H2O <> H2CO3. That is to say, as H2CO3 concentration is increased, some of this H2CO3 will transform to CO2 and H2O. Since CO2 will want to maintain equilibrium with the atmosphere, some of this CO2 will be lost to the atmosphere, and hence, as H2CO3 concentration in the water is increased, the end result is an increase in H2O (water).

So we can see the the reaction rates of the transformations between the various chemical species is actually very predictable. We only need to know the maths about the transformation rates if we like maths. Really, all that we need to understand to maintain balance, is that the rate of transformation of the chemical species is constant, and only changes when the balance between the chemical species is significantly altered.


TLDR: Add the 15 grams to your filter, without the addition or subtraction of other substances to your aquarium, if pH rises you added to much CaCO3, remove some. If pH lowers, you haven't added enough CaCO3, add more.


Quote:
Originally Posted by Positron View Post
Right! So I stopped using my pH probe to control CO2 a few years ago. It took me awhile to realize that my actual CO2 levels in the aquarium would fluctuate independent from my pH. Doh! changing KH values again.
Indeed. Where we inject CO2 at a rate greater then that from the reaction of CO2 + H2O <> H2CO3, KH does not affect CO2, but does affect pH. In this situation, the concentration of CO2 in the water is controlled directly and solely by the rate of injection of CO2, and the surface area of the water.

If KH drops, CO2 concentration does not change, but pH will decrease. So with a pH controller controlling the supply of CO2, the supply of CO2 will drop as the controller thinks CO2 concentration has increased with decreasing pH.

If KH rises, CO2 concentration does not change, but pH will increase. So with a pH controller controlling the supply of CO2, the supply of CO2 will increase as the controller thinks CO2 concentration has reduced with increasing pH.

As the surface area of the water is increased (increased surface agitation or whatever), the rate at which CO2 reaches equilibrium with the atmosphere increases, which will reduce the concentration of CO2 in the water, and increase pH. So in this situation, the pH controller will make the correct change to CO2 injection since the increase in pH was a direct result of the reduction in CO2 concentration.

As the surface area of the water is reduced (decreased surface agitation or whatever), the rate at which CO2 reaches equilibrium with the atmosphere decreases, which will increase the concentration of CO2 in the water, and reduce pH. So in this situation, the pH controller will make the correct change to CO2 injection since the decrease in pH was a direct result of the increase in CO2 concentration.

These are important equilibrium's to understand. Since for instance, if you increased the surface area of your water, the pH increases as a direct result of CO2. A pH controller in this instance will make the correct changes since it was the supply of CO2 that changed. But if you maintain a constant supply of CO2 without a pH controller, and determined that KH was increasing, and thus reduced KH, you have made the wrong determination. Here, you have maintained the same pH for a reduction of CO2 and KH.

I've gone a little overboard on the explanations, since I would like to be able to explain these processes easily for the creation of a topic that explains these processes in great detail in a simple manner, and I need the practice.

Feel free to edit.
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