Confused on PH, KH, GH, Carbonates, Hydroxides

6 posts in this topic

I have been trying to wrap my head around the differences between PH, KH, GH and how carbonates and hydroxides change them. 


As I understand it, carbonates like potassium carbonate and calcium carbonate raise PH, but they also raise alkalinity (also called KH) and general hardness (also called GH).  It seems the word "buffer" is used interchangeably for raising PH as well as raising KH, but I would think it is more properly used when describing KH, but it's extremely confusing reading what some people say on blogs and youtube when they use the word "buffer" because of the differences in carbonates and hydroxides.  I would think you "buffer" KH and you "raise" or "lower" PH.  I admit, this is from my years as an aquarium hobbyist, so maybe I'm off here.  Can someone give me some clarity on this please? :)


I've also read a lot of conflicting information on the need for carbonates in the system water.  Nate Storey claims the presence of carbonates are problematic and best avoided.  Rupert of Oz and most other people say carbonates are essential for the beneficial bacteria and must be amended from time to time, or you will eventually have a collapse of your bacteria.  Both of these guys are extremely smart people, and I respect both a lot, but how can both be right?  I'm confused. :)


As I understand it, Hydroxides raise PH and GH but not KH.  If that is the case, why do experts like Lennard recommend and use hydroxides for PH control?  I would think that there would be drastic swings in PH throughout the day if the system water is lacking KH, unless the hydroxides are used in a continuous drip system, like a kalkwasser drip.  It seems like hydroxides are excellent for using to adjust and dial in PH, but carbonates would be needed to keep the PH from quickly declining after treatment.  Am I making the wrong assumption? :) 


Thanks for any input you can provide.  I love this forum!



Jarrad and kellenw like this

Share this post

Link to post
Share on other sites

Thanks for replying Rupert.  This is starting to make more sense now.  I'm trying to figure out this quote thing so I hope I don't mess it up too badly.



The use of carbonates or hydroxides raise the ph alkalinity.... kh & gH are often referred to or measured in degrees of "alkalinty", or more correctly Calcium Carbonate

This has always been a source of confusion for me. It seems like "buffer" and "alkalinity" are used in so many different contexts and yet sometimes in the same, that it's always hard to tell exactly what someone is trying to really say. So, is it best to say "carbonate hardness" for KH, "mineral hardness" for GH and "alkalinity" or "acidity" for PH? Where does "buffer" really fit in there?


They refer to the ability to buffer pH...

I so often hear people say "buffer" for KH and PH. Is it appropriate to use the word "buffer" for both?


Carbonates will raise kH in conjunction with pH... and could raise gH.. if used in very large quantities... but both your pH and kH would be off the scale, and your fish and plants probably dead... :D

General hardness (although expressed in degrees of carbonate)... is a measure of the "mineral" content of the source water... (usually described as "hard" water)... and usually reflect Calcium, Magnesium and/or Iron.... and any others

So carbonates mainly raise PH and KH but pretty much have minimal impact on GH? I figured that they would raise GH too, if using potassium or calcium carbonates, since my assumption is that GH is "mineral hardness" and there's a lot of Ca or K in those carbonate compounds. I may be confusing myself here.


Nate's video was somewhat confusing.. and taken out of context... he was referring to how high carbonate content in high pH/hardness source water systems can be problematic.. as often seen.... and frequently posted..

Like as in nutrient lockout? It seemed like he was saying he preferred to avoid adding carbonates though, not just voicing concerns about source water with high carbonates.


It's not so much the "carbonates" that are esential in a low pH situation.. such as when buffering is required.... but more the provision of a "carbon" source for the nitrifying bacteria...


In low pH scenarios... (which by definition essentially mean very low, or near nil carbonate buffer capacity)... the bacteria don't have the carbonate as a carbon source... (as they would in a higher pH).. instead utilising the CO2 in the system being produced by the fish.. which otherwise would form carbonic acid.. and drive pH even lower... (again related to the fact that there is no buffer capacity)


 This rings a bell with me. I used to have a 110 gallon planted aquarium that I dosed CO2 in. It was always very low in KH, but maintained acceptable GH and a stable PH in the 6.4-6.8 range. I also added Flourish, which is a carbon additive. I frequently removed and rotated plants in and out of the aquarium. Sometimes I had zero plants in the aquarium since I frequently "aquascaped" it just for fun. Nitrates were always pretty high, but never ammonia and nitrates never registered, so I assumed that the CO2 and carbon additive kept the bacteria propped up. Does that sound right?


People suggest "hydroxides" for pH control... as commonly used in RAS... for the reasons posted above... and because that leaves the pH within a preferred range for plant nutrient uptake... 6.2 - 6.8...


Others prefer "carbonate" compounds.. or even quantities of "shell grit" or even limestone... to maintain both a more "stable" pH... but also a more "neutral" pH closer to 6.8 - 7.4...

Diurnal swings in pH are more a feature of algae... and significant in pond based aquaculture... but negliable in most backyard aquaponic systems...


But yes dosing with excessive hydroxides (or carbonates for that matter) in a single dose could certainly result in large pH swings... and the "Kalkwasser" drip methodology was invented as a means of constant pH buffering (equated to feed levels).. for this purpose...

This is where I start to question why we'd use hydroxides in aquaponics. It seems like carbonates are easier to dose, less reactive and provide better stability over the longer term. Is this a bad assumption? But, maybe it's better to dose hydroxides daily during feeding instead of trying to dose carbonates weekly or bi-weekly?


Many ornamentals, corals and marine crustacea, molluscs etc... require a "hardness" content.. that's more related to Calcium for shell integrity...

This may be a silly question, but could you theoretically control PH with hydroxides and provide the "hardness" required by crustaceans for shell development with calcium chloride?


And as such it's "normal" for aquaria circles to recommend "minimum" kH levels... but that's also in conjunction, even if not expressed... with pH ranges above what is typically used with freshwater fish... and certainly in AP systems...

I assumed that I should try to keep a PH pretty close to neutral in my AP system. Would it be better to be slightly acidic instead, for nutrient uptake?

I'm really sorry for all these silly questions. Thanks for taking the time to help this newbie out. :)

Edited by Stephanie (see edit history)

Share this post

Link to post
Share on other sites

Stephanie, seems the problem you are having is a lack of definition.


I will do my best without getting geeky. ;)



pH is the measure of hydrogen cations (H+) within the water. pH lower than 7 has more H+ (i.e., acidic) and pH above 7 has fewer H+ (i.e., basic or alkaline).


Alkalinity is the measure of water's ability to resist a change in pH. This "resistance ability" is referred to as the buffer.


Buffer is a compound that creates resistance when changing water's pH due to the compound maintaining a balance of H+ at a particular pH. All buffers have their own inherent pH as well. This inherent pH of buffers is what determines how the buffer will change the pH until a point, that is the buffer stability point. Once the stability point is reached, a higher alkalinity will no longer change the pH.


KH is the measurement for carbonate hardness. However, this is not correct in our context. Technically, due to the testing methodologies used in most hobby test kits, KH is a measurement of alkalinity and not carbonate hardness. The terms KH and carbonate hardness is really quite obsolete except continues to be used amongst hobbyists. When people say alkalinity, then they are talking about KH and carbonate hardness. Do not confuse carbonate hardness with GH (general hardness) even though they can be both shown as a measurement of calcium carbonate.


GH is the measure of all divalent or bivalent ions, which are most minerals (i.e., calcium is Ca++, magnesium is Mg++, copper is Cu++, ferrous iron is Fe++, and many more). However, for us, GH is primarily dominant in calcium and magnesium. There are minerals that can act as a buffer to an extent, such as calcium when carbonates are too high, but this resistance is quite weak thus GH should never be viewed as alkalinity.



Most common and cheapest buffer hobbyists use is the carbonate bicarbonate buffer. This buffer has an inherently high stable buffer pH point thus, when using anything like carbonate or bicarbonate products, then the pH will go up as your alkalinity goes up. This is not the same as saying that pH will always go up as the alkalinity goes up since this is dependent on the buffer's pH stability point. Potassium phosphate buffer system can actually have a higher alkalinity at a lower pH when compared to the carbonate bicarbonate buffer system.



Carbonate/bicarbonate products do significantly increase performance of nitrification microorganisms. However, as carbonate/bicarbonate concentration increases, then pH will change. The carbonate/bicarbonate buffer pH stability point is around 8.2~8.4ish (depending on many variables). Increased nitrification performance is one reason why fish prefer a higher pH, except of course this is species dependent.


Now, whether you should aim for a higher pH or a lower pH, this will be determined by your fish and plants that you want to grow... Some plants do fine at high pHs and other plants struggle at high pHs. So, since you are doing a single loop system, you must compromise; the compromise is around 6.5~7.2 (give or take).



The reason hydroxide solutions are used is that hydroxide solutions are incredibly cheap; however, their potency is not forgiving if you screw up. Wood ash is actually a potassium hydroxide product, except also contains all sorts of other stuff (which you can filter out). Carbonate products are not as potent as hydroxide solutions, but still are not very forgiving if you screw up. Bicarbonate products are very soft and not very potent, but, if you try screwing up hard enough on the measurements, then they will bite you with a high pH. When considering carbonate/bicarbonate products, dissolution rate should be considered. Calcium carbonate (i.e., crush oyster, shell grit, crushed coral, aragonite, etc.) is extremely forgiving due to having a very low dissolution rate. Potassium bicarbonate is less forgiving than calcium carbonate due to having a very high dissolution rate. Potassium carbonate has a higher dissolution rate than calcium carbonate, but this will raise the pH much faster than potassium bicarbonate.


Hydroxide is a hydroxyl anions (OH-). Hydroxyl anions is not a divalent cation so not possible for hydroxide to increase GH. However, the counterpart attached to the hydroxide could increase GH, such as Calcium Hydroxide (which is one Ca++ and two OH-).



Mr. Oz is correct about Dr. Nate's statement about carbonates. The statement is often taken out of context and Dr. Nate was talking about carbonates being avoided at high level. In that same video, I believe Dr. Nate also recommended a carbonate product such as dolomite, which is calcium magnesium carbonate.



Hope this helps. :)

Edited by crsublette (see edit history)
ande and TheDictator like this

Share this post

Link to post
Share on other sites



The relationship between alkalinity and pH is one area where most aquaponicists really struggle for understanding.   While I can manage pH from a practical perspective, I still lack the ability to provide a cogent explanation of what's happening.


Arguably the most frequent problem that people encounter is where their source water has a very high pH and they lower it using acid - only to find that it bounces back up again with hours.


I'd like to express my appreciation to Rupert and Charles for their efforts to bring understanding to this very important subject.



Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now