Here is the best explanation I found out there. I didn't write this.
I just started doing kalk/vinegar, so I personally don't have any long term experience with it.
"When Calcium Hydroxide solution (Kalkwasser) is slowly dripped into your aquarium, it captures free Carbon Dioxide present in the tank water and converts it to Bicarbonate ions (which is a good thing), like this:
Ca++ + 2(OH-) + 2(CO2) <==> Ca++ + 2(HCO3-)
If you drip too fast or if there is not enough Carbon Dioxide available in the water, your shiny new Bicarbonate ions will be converted to Carbonate ions (a bad thing), like this:
Ca++ + 2(OH-) + 2(HCO3-) <==> Ca++ + 2(CO3--) + 2 H2O
The Carbonate ions formed will make the Ca++ you are trying to add to your tank get wasted by the useless precipitation of Calcium Carbonate -- the white stuff you are seeing.
So, too rapid addition of Kalk may actually cause the Calcium and Alkalinity in your tank to go DOWN instead of UP (a bad thing), like this:
Ca++ + 2(HCO3-) + Ca++ + 2(OH-) <==> 2 CaCO3 + 2 H2O
In the above reaction, a Calcium ion and two Bicarbonate ions from the aquarium combine to form solid calcium carbonate -- the white stuff you are getting in your tank, which is really just a kind of sand.
This can happen even with a slow drip of Kalk if there is not enough CO2 in your water -- something you can't easily control.
To avoid this, try mixing and adding your Kalkwasser like this: pour 15ml of 5% Acetic Acid (or ordinary Distilled White Vinegar from the grocery store -- same thing) into a 1 liter (1 quart) container. Dissolve 1/2 teaspoon of lab-grade Ca(OH)2 (or commercial Kalkwasser mix) in the Acetic Acid, and then dilute to 1 liter (1 quart) volume with either RO/DI water, or even tank water.
15 ml is more Vinegar than some people are comfortable with, but I use it constantly with no problems. There should be no sediment in the mixture, or just a little bit at most. You can let the sediment settle out if you don't like the white flakes in your tank. I just drip the liquid and the sediment both into my tank.
Dissolving the Kalk powder in the Vinegar first will accomplish several very good things.
First, it will get more Calcium ions (Ca++) into the solution because you are dissolving the Ca(OH)2 in an acid instead of water, and forming Calcium Acetate, which exists as a dissociated equilibrium of free Calcium ions and Acetate ions.
Second, the Acetic Acid (Vinegar) provides an equivalent of all the CO2 you need to avoid precipitating the newly-added Calcium ions as useless white Calcium Carbonate powder.
Third, after all the cool Calcium ion chemistry is over, the leftover Acetate ions from the broken-down Vinegar leaves you with free organic Carbon in the water that feeds the bacteria in your tank so that it converts more poisonous Nitrates to NO2 gas (a very good thing).
Adding Vinegar in Kalkwasser is one of the few win-win situations for reefers -- it has a great up side and I've yet to encounter a down side to doing it. I don't know why so few reefers do it -- lack of understanding of the chemistry behind it maybe -- but a lot more are starting now that some respected reef writers have discovered it and have recommended it and even written up detailed instructions for it.
By the way, you should check your pH before and after you do this the first few times to make sure it is not affected by the process. It should not be a problem. Also, if you don't already have them, get and learn to use Salifert test kits for Calcium, Alkalinity, and Magnesium. The levels of all of these are related and affected by dripping Kalk.
Good luck."
