General Hardness- Part3
One of the easiest ways to increase water hardness is to add a small filter bag of aragonite or crushed coral (substrates marketed for marine aquariums) to your filter. The substrate will slowly dissolve, adding a steady source of both hardness and carbonate hardness (discussed later) to your water. You can also add these substances to your substrate, but hardness is harder to control that way. If the substances are added to the filter, you can always remove them or reduce the amount if you find that water is getting too hard. This method is best used in aquariums not receiving carbon dioxide (CO2) enrichment.
The acidification of the water by CO2 and the rapid water flow through the filter may cause the aragonite to dissolve too quickly, increasing the hardness and the buffering capacity (alkalinity)...