Typical home water softeners soften water using a technique known as “ion exchange”. That is, they remove calcium and magnesium ions by replacing them with sodium ions. Although this does technically make water softer, most fish won’t notice the difference. That is, fish that prefer soft water don’t like sodium either, and for them such water softeners don’t help at all. Thus, home water softeners are not an appropriate way to soften water for aquarium use.

Fish stores also market “water softening pillows”. They use the same ion-exchange principle. One “recharges” the pillow by soaking it in a salt water solution, then places it in the tank where the sodium ions are released into the water and replaced by calcium and magnesium ions. After a few hours or days, the pillow (along with the calcium and magnesium) are removed, and the pillow recharged. The pillows sold in stores are too small to work well in practice, and shouldn’t be used for the same reason cited above.

Peat moss softens water and reduces its hardness (GH). The most effective way to soften water via peat is to aerate water for 1-2 weeks in a bucket containing peat moss. For example, get a (plastic) bucket of the appropriate size. Then, get a large quantity of peat (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 is 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 in the filters of their tanks, the technique has a number of drawbacks. First, peat clogs easily, so adding peat isn’t always effective. 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 chemistry changes when new water is added (it has the wrong 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 a R/O unit. Unfortunately, R/O units are too expensive ($100-$500) for most hobbyists. R/O water can also be purchased at some fish stores, but for most folks the expense and hassle are not worth it. The same applies to distilled water purchased at grocery stores.