Perhaps you have soft water due to a water softening system in your home or your water is supplied by a well instead of the city utility. This could pose a problem if you're interested in keeping fish (and snails) that require harder water, though if you're new to fishkeeping it's best to select fish that can tolerate your existing water parameters versus trying to tinker with the water chemistry. But if you have your heart set on a cichlid tank and soft water stands in your way, you do have some options.
First off, let's look at what exactly constitutes hard water. The term "hard water" usually refers to the general hardness (gH), which is the amount of dissolved calcium carbonate (CaCO3) in the water supply (from limestone most likely). Carbonate hardness (kH) refers to the buffering capacity that keeps the pH from fluctuating. The larger the KH, the more resistant to pH changes your water will be.
Water hardness follows the following guidelines. The unit dH means "degree hardness'', while ppm means "parts per million," which is roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm CaCO3. Most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not necessarily mean it actually came from CaCO3.
General Hardness Scale
0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
Higher : liquid rock (Lake Malawi and Los Angeles, CA)
**Note: The following measurements are approximate; use a test kit to verify you've achieved the intended results. Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.
To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. You can add some sea shells, coral, limestone, marble chips, etc. to your filter to accomplish this. You would use this method to increase your water's overall hardness.
To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), a.k.a. - common baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. You would use this method to increase the buffering capacity in your tank to stabilize the pH without necessarily hardening the water.
How to soften your water
An effective way to soften water via peat is to aerate water for 1-2 weeks in a bucket containing peat moss. For this method, you would get a plastic bucket of the appropriate size. Then, get a large quantity of peat (a gallon or more), boil it (so that it sinks), stuff it in a pillow case, and place it in the water bucket. Use an air pump to aerate it. In 1-2 weeks, the water will be softer and more acidic. Use this aged water when making partial water changes on your tank.
Peat can be bought at pet shops, but it may be expensive. It is much more cost-effective to buy it in bulk at a local gardening shop. Read labels carefully! You don't want to use peat containing fertilizers or other additives.
Although some folks place peat directly in the filters of their tanks, this technique has a number of drawbacks. First, peat clogs easily. Second, peat can be messy and may cloud the water in your tank. Third, the exact quantity of peat needed to effectively soften your water is difficult to estimate. Using the wrong amount results in the wrong water chemistry. Finally, when doing water changes, your tank's water chemistry changes as new water is added, throwing off the desired properties. Over the next few days, the chemistry changes as the peat takes effect. Using aged water helps ensure that the chemistry of your tank doesn't fluctuate while doing water changes.
Hard water can also be softened by diluting it with distilled water or R/O water. R/O (reverse-osmosis) water is purified water made by an R/O unit. Unfortunately, R/O units are too expensive ($100-$500) for most fishkeepers. R/O water can also be purchased at some fish stores, but for most folks the expense and hassle isn't worth it. The same applies to distilled water purchased at grocery stores.
Information for this blog post was borrowed from Beginner FAQ: Practical Water Chemistry.