Aragonite

Calcium carbonate has many forms but generally fall into calcites and aragonites. Aragonite is the most soluble form of Calcium carbonate which is formed in several ways. The one most useful for pH, kH and GH maintenance is oolitic aragonite which has precipitated out of pristine tropical seas when the pH is very high due to photosynthesis. As the pH increases calcium solubility decreases causing a very clean and soluble mineral to form litte spherical egg shaped granules called oolites. This material start dissolving at 8.2 and increase with lower pH, salinity and temperature. Calcites are generally much less soluble and usually start dissolving at 7.2 pH. Dolomite is ½ calcium and ½ magnesium but is highly insoluble.

Oyster shell is primarily calcite, the significantly less soluble form of Calcium carbonate than aragonite. There is some aragonite in oyster shell but it is much less soluble due to the complex organic molecules such as conchiolin, perlucin and other proteins and polysaccharides that form layers of organics and calcium as the shell is formed. Oysters grow in nutrient rich waters and this is transferred to the shell. Oyster shells may not cause an out break of algae in a pond with the complex nutrient rich ecology but in a reef aquarium using oyster shells as a calcium reactor media will lead to outbreaks of diatoms and other pest algae. This is due to the organic material in the shell and the nutrient rich environments that they grow in.

“The oyster creates its own environment by secreting a shell composed or ninety-five percent (95%) of calcium carbonate. The remainder of the shell is made up of organic material and trace amounts of manganese, iron, aluminum, sulfate and magnesium.” http://www.yale.edu/ynhti/curriculum/un ... .02.x.html

Aragonite is 2x times more soluble in seawater at 25C than calcite. Calcite is most soluble in salty high pH warm water but is still out competed by aragonite. Aragonite is just the opposite and is most soluble in freshwater, lower pH and temperature which makes it ideal for maintaining water quality.
Morse J.W., Mucci A., Millero F.J.
The solubility of calcite and aragonite in seawater of 35%. salinity at 25C and atmospheric pressure (1980) Geochimica et Cosmochimica Acta, 44 (1), pp. 85-94.

6. This demonstrates that the solubility of aragonite is greater in a solution with little or no other salt than in a solution well supplied with other salts, and that it is greater in a cold than in a warm solution. Calcite is exactly the reverse.
Bulletin of the Geological Society of America, Volume 25 P751

The best form of aragonite to use is oolitic which has precipitated out in tropical seas (the bahamas) so it is very soluble and has some trace minerals in it. Oolitic aragonite is round and fluidizes with very little flow. My video show the reactor only expanded about 10% of the static bed so even though it's moving around a lot it just a small amount of flow. You can run reactors up to 100% of static bed height. Though it is more expensive (about a buck a pound) it is clean, works quickly and dissolves cleanly. Because it is 2x as soluble or more than calcites a much smaller amount can have the desired effect on the water quality parameters and over a much shorter time. It is a great substrate for the fish too. If you put it in as a sand your pH will always be 8.2 and your kH and GH will be very high.

The Caribsea Crushed Coral which is just mined aragonite the size of typical aquarium gravel. It can be put in box filters or media baskets. It works just as well as the oolitic sand but does not have the same self cleaning dynamics of a fluidized bed. So the gravel can trap some detritus but that is easily resolved by cleaning.

Goldfish are often listed as having a GH requirement with a low end of 5 dGH. In most scale this is considered soft but some considerate it moderate. We all know goldfish can survive and even thrive on water with 0 GH provide the pH doesn't crash the nitrogen cycle. If Goldfish were not adapted to soft water most garden ponds in the Northwest which usually have a <6 pH and 0 GH or KH would not have reproducing populations. There are even wild populations in very pure high elevation lakes.

In Fish Medicine by Michael K. Stoskopf he discusses that many fish can handle a wide variety of hardnesses but it is the lack of calcium ions that increases ammonia toxisity most likely by reducing plasma sodium efflux at the gills. The causes osmoregulatory stress on fish in soft water with high levels of ammonia. So if we have soft water that is poorly buffered, the pH drops below 7, the nitrogen cycle shuts down, ammonia builds and the fish do poorly. Aragonite fixes these issues by keeping your pH, kH and GH up. He goes on further discussing that the Calcium ions are also protective to the gills by reducing copper and zinc toxicity by competing with the metals for branchial adsorption sites, decreasing the metal uptake. Magnesium helps with this to but necessary magnesium and potassium levels can be maintained through food.

I maintain a 340 gallon Tanganyikan community tank with 4 to 6 species of fully reproducing populations of cichlids. At one end of the tank I have the smallest cichlid Neolamprologus multifasciatus breeding within the same two square feet as one of the largest cichlids in the lake if not the world Oreochromis tanganicae. They have a substrate of aragonite and beach sand. I do large water changes without adding any salts and they do very well. I’m on the 4th or 5th generation on the multifasciatus. There are few fish we keep with more stringent hard water requirements as wild Tanganyikan cichlids and mine thrive with just aragonite.

As with everything your milage may vary but aragonite is a simple way to keep things simple especially for those with soft water or limited understanding of water quality parameters. If you feel you mush add a bunch of salts there are several commercial additives such as equilibrium or even simply instant ocean if your system can handle a little good old sodium chloride.