The relationship with KH and pH.

KH is specifically a measurement of the carbonate ion: HCO3-
That’s it. Nothing else. No whams, no bams. Just carbonate. That simple. When you talk in ppm of KH, you are talking about parts per million of carbonate in your solution (in an aquarist’s situation: water).

So, then how does this affect pH? Well, you have to remember that pH is a measurement of H+ ions in the water. Each hydrogen ion, stripped of an electron, is looking for a pal. These ions, when measured, and then calculated, give you your pH.

What changes the number of free (yet lonely) H+ ions in water is something that WANTS these ions. Now, who might be hanging around, carrying a negative charge, wanting attention? You’re right: carbonate.

Carbonate accepts a Hydrogen Ion (H+) to make H2CO3. This is what is known as a buffer. This buffer will break up again to make H+ and HCO3-. Then it will combine again. This happens, over and over again. Hundreds upon hundreds of times.

The frequency (number of times it will happen, per minute) depends on a few things:

1) Number of H+ ions compared to the number of Carbonate ions (HCO3-)
2) Temperature (this can aid in chemical reactions)
3) Other buffers competing for the same H+ ions (which we will ignore, for now, until the end. It’s subtitled, please don’t use phosphates or other chemicals).

[b]…number of H+ ions….[/b]
I always whine at people not to use the instant “pH down” products designed to lower tank pH from a nasty 8.0 to a nice, cozy 7.0. First, as many of my peers have shown, fish can be raised, and raised well, in an alkaline pH. But beyond that, there are water chemistry issues. This would be why.

First, straight off, just consider the hazard to both you, your family, and your fish. Look at how [i]tiny[/i] that bottle is. Look at how [i]few[/i] drops it takes to treat so many gallons. Now, considering the amount of H+ ions it takes to make that pH change (quite a few) and considering the greater the number of H+ ions the greater the acidity…. within that bottle is a nasty, nasty chemical. It is most likely a form of sulfuric acid. It is not friendly. It will hurt you. It will hurt your fish. If you add that acid, and the fish are in its path, it will hurt your fish.

If adding that acid goes well, the common problem now is that by overloading your tank with H+ ions, you are reducing your buffering capacity by altering the frequency of this reaction [carbonate bonding with H+]. By having too much of your buffer tied up balancing these H+ ions, it can’t manage the nitrate and organic acids you want your buffer to neutralize. Your fish produce organic acids, causing your tank to get more acidic as it ages (old tank syndrome). This is one of the reasons we do water changes. Until we do that water change, the buffer is what helps get rid of those H+ ions, stabilizing the pH. If it is too busy absorbing all the extra H+ ions, the carbonate can’t help you by buffering.

Often, people consider that “…if I have 100 ppm of KH, and I want it to 60… and I reduce it down to that by overloading it with 40 ppm of acid (H+).. the net result is the same.” Unfortunately, it isn’t. The best way to think of it is get a room full of 100 little boys. Make them square dance with forty little girls in a small gym. Then throw in other girls, increasing the amount over time. Chaos. People are bumping into each other, wrong hands are everywhere…. what a mess.

In contrast, if you only had 60 little boys, standing there, looking forelorn…. those little girls would know exactly who to chase, and there’d be more than enough room for the ensuing melee.


Temperature makes electrons move. Makes them dance. Remember those children I mentioned? Well, let’s think of them high on sugar now. Yes, their pace is increased, running around like maniacs. If your pH is perfectly neutral when hands are being held, and the number of hands being held and length of time the hands are held can be aided by increased temperature. Normally this is not enough to cause drastic changes in tank pH, but it is a consideration overall. Mainly because sometimes, when H2CO3 forms – it [i]can[/i] gas out as H2O and CO2. When that happens, you lose a point of your buffering, as well.

Temperature can also change your pH by increasing your amount of solids per water through evaporation. HCO3 is left behind, your water evaporates off… this changes your parts per million of HCO3-.

[b]…other buffers…[/b]

Oh, no, you’re not at the end yet. I thoroughly enjoy listening to myself rant about my limited water chemistry knowledge.

To sum up all of this – what makes HCO3- dictate your pH is [b]rate[/b]. Not frequency, but [b]rate[/b]. The rate of a carbonate ion accepting an H+ ion, compared to the rate of H2CO3 losing an H+ ion.

[b] … rate? RATE! You were talking about frequency… [/b]

Yes, I was. The two are related. In order to achieve the perfect rate, you have to either increase or decrease the frequency. Think of rate as the speed you want to go on the highway, and the factors altering frequency (listed above) as the brake and gas pedals.

Why is rate important? Because it dictates the number of free H+ ions in your water. This is when you will have the most stable number of H+ ions in your water. This is equilibrium. And when certain chemicals are very good at maintaining this equilibrium (making their rate shift one way or another to accomodate slight abundances), they are known as buffers.

When chemicals buffer like this, they will maintain a certain pH. This makes sense, because buffering dictates how many H+ ions are available, and pH is the measurement of those H+ ions (a different type of measurement than ppm, but it is the same – the number of hydrogen ions per unit of solution).

For carbonate ions (HCO3-), which are excellent buffers, they LOVE to buffer at 8.0 to 8.2. They are very good at buffering at that pH. Just a little carbonate will gulp up free H+ ions, and this causes alkalinity (a lack of H+ ions). To stop the carbonate ions from gulping up too much H+ (and keeping a respectable pH of 7.0 ) we restrict the number of bicarbonate ions, and they are always hungry for H+ ions. This makes them an excellent buffer for our aging tanks…. as long as they don’t occur in too much quantity.

[b]…so…you were going to talk about phosphates?[/b]

Yes, I was. Because you were going to figure out that carbonate is only one of what must be many, many buffers. So why can’t we use any others that are closer to an equilibrium pH of 7.0?

Well, first is the aqueous solutions and toxic chemicals. That aside, the one chemical that does like to buffer at a pH of 7.0 and is aqueous and non toxic is Phosphate. This will balance your tank rather nicely at a pH of close to neutral, however, by means of a similar chemical reaction.

The reason I caution about phosphates is simple – when using phosphates to buffer your tank, you can no longer figure out your proportional chemistry to know your CO2 rate for your plants. It can also invite algae problems, as phosphate (not carbonate) is a favorite algae nutrient.