Water hardness can be most simply described as the mineral levels in the water. Hard water has a high dissolved mineral content. Soft water has very little. The most common mineral in water is calcium; however, other minerals may also be present. Most people’s tap water is either slightly hard or soft depending on where it comes from. Well water from areas which have a lot of limestone (calcium) is often hard. Water that comes from lakes and rainwater is often devoid of minerals, making it soft.
It is important to understand how water hardness affects pH in your aquarium. Hard water (high mineral content) is usually high in pH. Soft water (low mineral) is usually low in pH. The mineral in hard water acts as a buffer that combats acidification in the water. The resulting water will be more alkaline and higher in pH. Some species of fish require hard water while others require soft.Without the correct Water Hardness, it’s virtually impossible to fix pH levels.
The problem arises when we try to lower the pH in hard water. If we add a commercial pH decreaser to an aquarium with hard water, the mineral in the hard water will buffer the water and make it difficult to successfully lower the pH. We first have to remove the mineral from the water before we can effectively lower the pH. To soften hard water, you need to take the mineral out with a natural “water softener”. Another option is to use demineralized or RO water for your fish tank.
The same is true for trying to raise the pH in acidic water that is soft and doesn’t contain much mineral. Until we add mineral to the water, it will be difficult to successfully alter and maintain a high pH level. So what do we do? The easiest thing to do is add mineral in the form of calcium-based rock, thus making soft water hard and more alkaline (higher in pH).