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So I got my system cycled by getting the kH above 40 ppm. I added a good amount of calcium carbonate and potassium carbonate to the system every day and the cycling seemed to take off once it was above 40 ppm.

Now I am trying to understand why I am still needing to add carbonates to the system every few days to keep the kH reasonable. This might sound like a silly question, but why is the kH even dropping over time? Where is it going? I thought it would stay in solution but it must be consumed by some chemical process? Or, is the calcium carbonate being removed from the water by building up on the top layer of media? The top inch of my media is coated with white mineralization...that looks like the chalky white calcium carbonate powder...

Maybe I am missing something, but should I really be adding a couple of tsp of hardness with each 5 gallons of water added to the system? The ammonia, nitrites, nitrates, pH, and temperature are all good as you can see at this link...

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Carbonates are an essential carbon source for the bacteria to multiply and grow. No carbonates and your system could crash. See RupertOfOz comment from September 1st here :

What is your kH reading at? I would think you could be adding it less frequently than every few days. Also check your source water you use for top ups. It may have plenty in it already and adding additional may not be necessary or at least not necessary to do so as frequently.

Here is another one...

Arwen, the real short quick answer is that for every 1mg of NH3/4 that is oxidized to NO3...your bugs 'consume' about 7mg to 8mg of carbonate alkalinity, which is why 'it is dropping over time'. Remember, nitrification is an acidifying process...and since your source water has a kH of dH-0, you need to intervene and supply that in some way (like you are doing).

Changes in pH are just an 'end product',  the mechanisms by which pH is actually lowered in an AP system (and in nature) is a product of the chemical equilibria of the various components of the carbonate cycle as they relate to bacterial respiration. There are voluminous tomes (literally) written about the carbonate cycle/system, and it's various components (CO2, H2CO3, HCO3 and CO3) it would be ridiculous (and quite impractical) to get into it here. It's a HUGE and very, very complex cycle/system with many, many back roads and of which go beyond my abilities to fully describe. But again, if you have a specific question which hasn't already been gone over, feel free to ask and I'll try and do my best to explain.

I think a lot of this has already been gone over in your other thread about cycling and your low pH...If you've re-read it and still have questions, just ask :)

In practical terms though, just keep your kH above 40ppm or you will begin to impair and/or even crash your bio-filter ir-regardless of where your pH is at (like if you were using only RO/rainwater and only hydroxides to raise pH...yes, you can crash your bio-filter even at a pH of 6.8. It is wholly and totally possible in a particular set and setting)...[Sidney Biesterfeld, Greg Farmer, Phil Russell, and Linda Figueroa Littleton/Englewood Wastewater Treatment Plant study (2001)]

You just happen to have 'the opposite' problem most folks have...which is too much kH. So, instead of adding it, they need to neutralize and get rid of at least some of it by treating their top up water with some sort of acid.

Your bacteria need an inorganic (mineral) carbon species food source in order to live and oxidize ammonia. This is provided by kH. kH also neutralizes (buffers) the acid the bacteria produce through aerobic respiration (this part of it is not exclusive to carbonates, as hydroxides will also neutralize acids...but WILL NOT provide the carbon food source required)...

Arwen, I just looked at your chart. You can stop making such frequent additions. Let your pH and kH fall a bit.

Also, keep in mind that your carbonate is about twice as powerful of a base than your bi-carbonate since the carbonate can neutralize two protons, while the bi-carbonate can only neutralize one. (I know this may seem counter-intuitive, but trust me).

Anyways, because of things like chlorinity and temperature, presence of algae, or even plant densities will 'greatly' (or at least appreciably) effect pK (one or more equilibrium dissociation constants) in the acid base (bicarbonate/carbonate - carbon dioxide) reactions going on in each of our particular systems...we each need to pay attention to our own to see exactly and in what amounts kH plays out in terms of pH. Let your kH drop and your pH will fall into a better range for plants. (IMO kH at 35ppm would be my ultimate low/safe cut off threshold before I long as pH was above 5.5 to 5.7). Because my particular systems water chemistry, I can have a lower pH than you have even at your kH of 53ppm). Let it drop to 40ppm or so and see how that plays out against pH in your particuar systems water chemistry. It will be easy for you to correlate, since you chart and graph everything so nicely.

You can add crushed coral to you system in mesh bags to stabilize your ph swings. It acts as natural buffer for your ph levels. This will also save you allot of time and money on having to add allot of Hydrated lime into the system to keep the levels up. It will buffer the ph level so that you will need less lime to keep the system stable.

You guys are awesome!! Thanks for the feedback. My kH just dipped below 40 ppm so I have brought it up again. My source water has a kH of 0 ppm....Denver City water...go figure. This answers my questions for sure. Now I understand that the kH is actually being consumed, and that I am in fact adding two inputs to the system; both fish food (pellets) and bacteria food (carbonates) to the system. The system is running nicely so no real problems just curious how it all works. Thanks again!!

Ok then. Now for the fun part...

If we now see that carbonates, in addition to their pH buffering properties...(which is just their ability to neutralize the acidic by-products of aerobic bacterial respiration), are essential at a threshold of 35-40ppm because they provide the necessary carbon food source for our bacteria...we can eliminate their duties as a "acid neutralizer" using hydroxides for that part. This frees up their role in the scheme of things to being a carbon food source.

This allows us to keep just enough carbonates around to keep our bacteria "fed" with the necessary mineral (read:inorganic) carbon source they that carbonates don't really impact pH a whole lot.

Since they won't be around in numbers to keep pH in the necessary range for bacteria to thrive...we can now use the stronger and more precise (and fast acting) hydroxide group. preferably potassium hydroxide (KOH).

This will of course mean daily monitoring and (hydroxide- pH) adjustments, (in addition to keeping an eye on kH). BUT, all this fuss will allow us to run a plant growing POWERHOUSE of a system (since it should allow us to now run our AP systems consistently in the upper 5 to very low 6's pH range).

This is what and how I believe Nate is doing things. (Though he's been curiously quiet on these details concerning the matter...Which I'm sorta grateful for. Otherwise, if he had just spelled it out,  I probably would have never bothered to research, read, experiment, crash, succeed, crash again, succeed 'better' with a better and/or fuller understanding of these things).

BTW...@ Wes Hydrated lime is a hydroxide, and won't really do a thing (at least directly) for your kH...only your pH. And using a bag of shell grit, while being a totally valid, awesome, and low key method to stabilize a systems pH, rather sucks if you want tight control over a system's ph and makes it impossible for the operator to run things at a nice low plant loving pH ( say, 5.7 to 6.2)...

So to re-cap...

You can use the carbonates only as a carbonate food source (35 to 40ppm).

Which will allow you to use hydroxides to keep your pH hovering above the threshold for a pH related bio-filter crash (pH 5.5)

Here is a 'fun' "needless complication" for those who want to take it so far... CO2 can also provide the necessary inorganic carbon source since it is essentially a major component of the carbonate cycle (in a given set and setting).

This is how people practicing (what I call) the 'Zen Garden" low input/low output model of AP do/are getting away with. But that has more to do with creating a truly balanced eco-system (anoxic and anaerobic zones et al) than it does running a high density agricultural production model of AP that churns out a consistent number of vegetables in a consistent and timely manner...

Which does not demean the value  or worth of such AP 'Zen gardens' one bit in my eyes...on the contrary. But it does make that model of AP less reliable or productive as a method of agriculture (and hence sales/profitability or food for your family).  

One model truly strives to be an "eco-system", the other an "intensive agricultural system", and the distinction must be made. (Particularly by those selling the idea of AP to the it seems unfair to hook people into the romantic notions of the one model, and in the same breath the entice them with the 'highly productive' entrapping's of the other...because the two, in reality, do not go together...except as an "AP marketing ploy")...

Thanks, Vlad. I suspected this, but it's nice to have these points articulated so well. 

Wow, are a wealth of knowledge. This sounds like sound advice. My pH is just starting to drop, so I will just add enough carbonate to keep it at 40 ppm, but no more. Hopefully my pH will drop to something better for the plants.

Also, I was wondering why you would use hydroxide instead of carbonate....and now I know. I'll manage the pH with hydroxide and feed the bacteria just enough carbonate like you suggest. I bought some "Hi-Yield" brand Hydrated Lime (CaOH2...that's the right stuff for only buffering pH right? 

Thanks again.

Yup that will do fine. Just be careful not to over do it. Hydroxides are strong stuff. You may want to make, or buy some potassium hydroxide (KOH) sometime down the line to alternate with.

As I move "up the scale" to more nutrient hungry plants, and running a lower pH, I'm finding that magnesium (Mg) is becoming the limiting factor. Since my top up water is very high in carbonates already, I'm not able/willing to use dolomitic lime (magnesium-calcium carbonate) as an Mg source. I've used both Epsom salt (MgSO4-7H2O), as well as magnesium chloride salt (MgCl-6H20) in some of these test systems successfully. Chloride or sulfate salinity has not become a problem yet, when used sanely/properly...not sure how that pans out in the super long-term. At the very low levels I use, I suspect that it would never become a problem...

So Vlad, among other things you're telling us to invest in Tums

Do you ever sleep? Just say'n.

Sure...and 'Phillips' for the hydroxides (milk of magnesia) 

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