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) of the water more than you want. If supplemental CO2 is used, then it works well to hang a fine mesh bag of aragonite in an area of the aquarium with less water flow and where it can be easily removed. There are also commercial products available that can be added with each water change to increase the carbonate hardness, general hardness, or both.
While hard water will, to some extent, limit the kinds of plants you can grow in your tank, aquarists in Europe successfully grow beautiful planted tanks in water with hardness exceeding 300 mg/L. even in areas with hard water, there are plants that can be grown successfully. It is just a matter of choosing the right ones. I still strongly advice that you learn to use the water you have before trying to change your tap water chemistry significantly, at least in the beginning.
If, after you learn what you can grow in your tap water, you decide that you really need to grow species that most have softer water, there are a few options. If you have a small tank, the easiest way to soften the water is to purchase bottled water at the grocery store. Figure out how much tap water you need to mix with the bottled water to achieve the hardness level you desire.
Some people collect rain water to mix with their tap water, but you need to have a good method of preventing unwanted chemicals from getting into this water, or it can cause more problems than it solves. Collected rain water also often attracts unwanted mosquitos to the yard.
If you need to prepare water for large or multiple tanks, you may want to invest in RO (reverse osmosis) or DI (de-ionization) systems. These expensive filtration systems are only for someone who is very serious about aquatic gardening and has a severe hard water problem. For most of us, learning to garden successfully with the tap water available is a much better option.
Source: Sunken Gardens/ By: Karen A. Randall