Where’s the DOWN VOTE BUTTON!!!! I’m just kidding. I have a pretty rough understanding of precipitation and assumed the precipitate crystals tended to reduce well before calcium carbonate starts to dissolve when it comes to lowered ph levels (8 and under) as noted in something like a calcium reactor (well below 8). From your explanation would that reduction be due to surfaces becoming coated by organics/mag/phosphates and essentially blocking the deposition of precipitation and essentially keeping it in suspension? If i’m not way off on this, lol, what happens to that precipitation?
Calcium carbonate (aragonite) is super saturated in normal seawater at pH 8. Assuming an alk of 7 dKH, it will not become able to be dissolved until the pH drops below 7.8. Somewhere below that it becomes unsaturated and can dissolve, as can coral skeletons, live rock, sand etc.
I don't think you can selectively dissolve the new precipitates, and all of the calcium carbonate surfaces might begin to disoslve, and worse yet, all those then become clean surfaces to act as seed crystals for additional precipitation when the levels rise again.
That's why I think the answer is poisoning of the calcium carbonate surfaces. That is what is known to happen in to ocean and why calcium carbonate is supersaturated and does not precipitate out.
I discuss the magnesium poisoning effect here:
One section from it that pertains:
Summary of Abiotic Calcium Carbonate Solubility Effects
This section summarizes many of the ideas covered in the above sections and puts them together to form a more complete understanding.
1. Normal seawater (calcium = 420 ppm, pH = 8.2, alkalinity = 2.5 meq/L (7 dKH)) is significantly supersaturated with calcium carbonate. That is, more of the ions (several-fold more, actually) are already in solution than would be stable in the long term. The rate at which calcium and carbonate ions land on a pure calcium carbonate surface in seawater is higher than the rate at which they leave that surface. This supersaturation sets up the potential for calcium carbonate precipitation.
2. The potential precipitation described in (1) above is "delayed," sometimes indefinitely, as magnesium gets onto the growing calcium carbonate crystal structure. The magnesium alters the surface, making it no longer look like calcium carbonate. This "poisoning" of the surface slows or stops the precipitation of additional calcium and carbonate onto it. Abnormally low levels of magnesium will be less effective at preventing the precipitation of calcium carbonate.
3. The more calcium and carbonate there is in excess of "saturation," the faster the potential rate of calcium carbonate's precipitation. In other words, the more the "on rate" exceeds the "off rate," the faster precipitation can take place. If the potential for rapid precipitation exists due to a very high supersaturation condition, the more likely such precipitation is to overwhelm magnesium's ability to prevent it.
4. The factors that lead to higher supersaturation are higher calcium, alkalinity and pH. The effect of pH is especially dramatic, with an increase of 0.3 pH units being equivalent to a doubling of calcium or alkalinity in terms of the supersaturation (or in terms of the driving force for precipitation). This pH effect is why an overdose of limewater can cause calcium carbonate precipitation, and why dosing limewater into a skimmer or other enclosed system (such as a pump intake) can increase precipitation of calcium carbonate inside it. It is also why reducing the water's pH in a calcium carbonate/carbon dioxide reactor can dissolve calcium carbonate media.
5. If the water is below "saturation" with respect to calcium and carbonate, then no net precipitation will take place. Under normal seawater conditions, where the water is, in fact, supersaturated with calcium carbonate, there is still little precipitation, largely because of the magnesium in the seawater. Consequently, if calcium or alkalinity is lower than "normal" in a reef aquarium, then boosting either (or both) calcium and alkalinity to natural levels will cause no rapid precipitation of calcium carbonate. In other words, boosting one under these conditions will not cause a rapid decline in the other.
6. When calcium carbonate precipitates, it uses up a fixed ratio of calcium and carbonate (1:1, or about 20 ppm of calcium for each 1 meq/L (2.8 dKH) of alkalinity). This ratio is the same as corals use to deposit their calcium carbonate skeletons. Abiotic precipitation of calcium carbonate, like coral skeletal formation, can incorporate other ions, such as magnesium and strontium. That incorporation will reduce the above ratio from 20 ppm calcium for each 1 meq/L of alkalinity to a slightly lower value. Over the long term this process can deplete magnesium and strontium in an aquarium if only calcium and alkalinity are supplemented.
