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.
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.