Adding calcareous substrate to the bottom of the tank is not, as you've discovered, a very effective way to raise the carbonate hardness. Calcareous material like coral sand stops dissolving once it is covered with algae and bacteria (think of how the chocolate in an M&M is "isolated" from the air by the candy coating).
Furthermore, the rate at which dissolution takes place depends on the speed with which the water passes through the substrate. Water hardly moves at all through a regular substrate. So the water around each grain eventually becomes saturated with carbonate ions, and dissolution effectively stops (yes, I understand it's an equilibrium reaction, but let's keep it simple for now). By contrast, in a tank with an undergravel filter, the water is constantly being pushed through the substrate. The water around each grain doesn't have long enough to become saturated with carbonate before it moves through the filter plate and then back into the main body of water in the aquarium. In other words, the undergravel filter ensures dissolution can continue at maximum speed all the time.
This is the illusion behind the coral sand substrate, and why there are plenty of stories of people with coral sand substrates but relatively low KH readings.
Marine salt mix (as opposed to aquarium salt) should raise the KH because it has lots of carbonate added. This does vary from brand to brand somewhat, so reading the ingredients on the boxes of marine salt in your local store might be worthwhile. You could also buy a box of Malawi salt mix, and add a quarter or half dose to each bucket of water. There are even "DIY"
recipes for making Malawi salt mix using cheap chemicals (Epsom salt, bicarbonate of soda, Lo-Salt, etc.).
On the other hand, a relatively low-tech, low-cost solution is to simply install and undergravel filter, ideally a reverse flow system coupled to your existing aquarium filter. By creating a two-layer substrate with crushed coral at the bottom separated from a coral sand surface layer with a gravel tidy, you can make a very effective chemical filtration system without any risk of anaerobic decay. This is, of course, "old school" marine fishkeeping, but it works well in fish-only systems, be they brackish water, marine, or Rift Valley cichlid ones.
The alternative is to stuff one of the "compartments" in your canister filter with crushed coral or crushed oyster. Since these things dissolve slowly, there's no risk of wild swings in pH or KH. Moreover, your weekly water changes will "reset" the levels each time. The whole point of buffering is that it doesn't keep getting more alkaline -- once the pH reaches a certain point (around 8.5) the calcareous material stops dissolving. But if the pH drops, dissolution starts up again, at a rate proportional to the acidity. And that's the point: it inhibits acidification, while not causing an excessively high pH either.
All this said, for brackish water fish you don't need to be too concerned either way. They're adaptable. These aren't coral reef invertebrates. So if your fish is happy and healthy, and water chemistry is somewhere within the tolerated range, then your fish will be fine. There's plenty of evidence to the contrary in fact: the times brackish water fish have been bred seem to be associated with people either doing water chemistry changes, or else keeping them in conditions that "sound" wrong. Monos, for example, only seem to spawn in freshwater.
One last thought: do also bear in mind live sand dies in brackish water. In the process, it decays, and that lowers pH and uses up the carbonate reserve in the water.
Cheers, Neale
