Aragonite, crushed coral, or oyster shell are sometimes employed for
KH and GH stabilization, however aragonite and more so crushed coral
& oyster shell (as with Wonder Shells) only aid to stabilize KH
(they are poor buffers, especially crushed coral) and should not be used
in place of a true KH buffer such as Sea Chem Alkaline Buffer when true
buffering is necessary due to fluctuating KH or pH whatever the cause
In fact way back in the early 1980s, there was a fad to use Oyster shells as a substrate in marine aquariums, not only did I find this use not only perform a poor job at maintaining alkalinity, mineral content was also not aided even as well as crushed coral (further investigation and experiments showed part of the problem was that oyster shell tended to pack down much more than crushed coral and allowed for organics to be trapped and decompose increasing acids far faster than the oyster shell could buffer).
Crushed Coral (as with seashells and coral) is primarily made up of
Calcium Carbonates (CaCO3) and has VERY LITTLE bicarbonates while
Aragonite is of similar make up, but has a much better surface area for
dissolving of minerals making it a better choice of the two (especially
when used in a reactor).
With Seashells/coral, the surface area is lower yet, even if in high flow areas of an aquarium and thus provide poor buffering if any (based on many studies in marine aquariums I have performed over the years)
Some Aragonites (that have high carbonate content) are useful at
stabilizing a higher kH of around 240 ppm, which is the maximum KH
(alkalinity) needed for Marine Aquariums, but does not respond to
changes rapidly enough when carbonic acids are produced at a rapid rate
in an aquarium (usually a high bio load or large amounts of organic mulm
will cause this).
Even in marine aquariums with aragonite, this may not always be enough to maintain a proper alkalinity (KH) level, especially in tanks with high bio loads and without adequate water changes (even skimming can remove some elements).
Aragonite generally has a few more essential minerals in higher concentrations such as the important mineral (for corals), Strontium.
Further Reference: Aquarium Bio Load
It often takes copious amounts of acids to free these minerals and
what little bicarbonates/carbonates that are available (which is where a
Calcium Reactor MAY be helpful in marine aquariums).
For this reason, the use of crushed coral is more effective in a “Filter Bag” to release these minerals when used in Freshwater, especially African Cichlid aquaria. The use of a filter bag in a high flow area will improve the dissolution rate releasing more minerals and allowing for some buffering (as well as slightly improved adding mineral cations), although again I will point out not much carbonate (KH) buffering due to the mineral make up of crushed coral (Aragonite will do a slightly better buffering job when employed in a filter bag).
This said, despite the popular use of crushed coral/Aragonite for pH/KH control in African Cichlids, it is a poor choice for this, especially in high bio load aquariums due to the FACT of its mineral make up.
Simply put, you CANNOT make a mineral appear out of nowhere that does not exist and that seems to be what many mistakenly believe when using crushed coral to increase KH/pH.
This is an unfortunate “aquatic myth” that many forums still perpetuate when a quick search of the chemical makeup of crushed coral would expose this folly.
And for those who use this equation to state otherwise: H2O + CO2 + CaCO3 <> Ca + 2HCO3, let me point out that this is great when used in a reaction chamber, but in the real world of an aquarium, I've yet to find that provides much buffering, including in marine aquariums with copious amounts of crushed coral. I still had to add additional buffers! Sorry actual experience with 1000s of aquariums trumps here!
The bottom line is that Crushed Coral MAY help maintain KH/pH in a
low bio load aquarium, they are best used for minerals (GH) and even
here Wonder Shells (or similar mineral ion supplements such as
AAP/SeaChem Replenish or Fresh Trace used regularly or in drip) are far
superior to Crushed Coral due to the fact a Wonder Shell dissolves at a
much faster rate and reacts much quicker to chemistry changes in the
water than does crushed coral.
As a well the use of a slow drip liquid mineral replenisher would also be superior to the use of Crushed Coral for mineral depletion.
Besides the simple mineral makeup of crushed coral, my own extensive tests show that its use to increase KH as well as GH (Calcium and other minerals) simply is poor.
Product Resource: SeaChem Replenish; Can be added directly or by drip