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Hi guys,

I'm new to this forum, i've tried to find a similar topic but i could not find it.
My crazy question is:
Is it possible to lower KH with Ascorbic acid (Vitamin C) ?
Lower KH means that the water is more acid, and Vitamic C is acid, no?
 

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Hmmm... you definitely can lower KH with hydrochloric acid- quite a lot of people use that method to reduce KH, but I’m not sure Ascorbic acid is going to be a strong enough acid to be effective...
 

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Is it possible to lower KH with Ascorbic acid (Vitamin C) ?
Lower KH means that the water is more acid, and Vitamic C is acid, no?
You could achieve this, to varying degrees and timing. Most of the idea behind adding acids to reduce KH/pH is to play the carbonate system. Carbonates are primarily what make up the KH in our tanks and are in different forms and quantities depending upon pH. See the “Bjerrum plot” for carbonates, below. You will see the pH levels where carbonates take different forms.

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So, as you will see in this plot, make the water acid enough and your carbonates turn into CO2, which is then gassed-off or consumed by plants/bacteria, but can be a slow process. As carbonates are consumed or driven off (as CO2), pH often rises again and this, in turn, raises KH as HCO3 re-forms. Carbonates are also bound to other elements such as calcium (CaCO3) or magnesium (MgCO3). As pH rises it starts dissolving these compounds again, if they exist. So, you can get a lot of back and forth as your system tries to find it’s balancing point.

It might be best to give you a crude example, which is more in tune with your purpose, and I am assuming that you are not injecting CO2 which, by itself will lower pH, but not KH directly. If you start with pH 7.6, your carbonate is HCO3 and will hold your dKH in the 4.0 area. Your objective is to lower pH and, ideally (so you don’t have to continuously add things) you want to remove most of the carbonates. To do this, you will have to move pH into the 6.0 area (great for plants and most tropical fish are happy there), where the HCO3 largely turns into CO2 (plot, above), and hold it there by adding acid until it no longer drifts up. In this CO2 form, plants will consume it and it will off-gas from your water, thus removing carbonates. There will be no bounce-back other than from whatever HCO3 is left. From there, you can add back carbonates to raise pH to your preferred level and it should stay in that area.

This is easy when RO is used for water changes and assumes that no new carbonates are being added via water changes, substrate, rocks, etc. Additionally, natural processes will gradually erode KH and cause pH to drift down, but offsetting this is now easy by simply adding sufficient carbonates. Of course, for most living in a different world where tap water is used, carbonate quantity (usually all HCO3 form) varies greatly, making adjustments more difficult.

While you could still target pH 6.0 by adding sufficient acids after a w/c, you now have to be a little careful about the potential to harm your fish with a TDS shock, e.g.; a 50% w/c with a 5 dKH change is going to increase TDS by nearly 50ppm. Depending upon your tanks’ TDS, this could harm your fish. So, in this case or if you use RO water, but want a higher pH than ~6 (plot, above), you will have to decide how much acid to add after a w/c and may benefit from dosing (dry or solution), along with your ferts, to maintain the desired pH, since carbonates (HCO3) will not be removed.

Further muddying the water, so to speak, can be things like CO2 injection (which can make pH/KH adjustments easier than the acid dosing approach) and substrates or rocks that are composed of various things to either raise or lower KH.

Yes, ascorbic acid can be a good choice to lower ph/KH. Strong acids such as Muriatic acid (HCL), Sulfuric acid, or milder acids such as or Fulvic acid will also work. One of the benefits of Fulvic acid, like many of the other humic substances, is that plants and fauna can benefit from it and you can control the dosing. Maybe a combination of ascorbic and Fulvic acids. Of course, diluting with distilled/RO water is a non-additive way to reduce KH. Some use peat, driftwood and Catappa leaves, although you can’t control these and Fulvic acid does the same thing but, as mentioned, in a controlled way.

Corrections and additions welcome.

cc: @Discusluv
 
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