From a homeowner’s perspective, very hard water leaves a residue on silverware, glassware, and appliances; deposits in pipes and faucets; and can make it hard for soap to do its job in laundry. Many people in areas with very hard water install whole house water softeners. These devices typically replace calcium carbonate in the tap water with sodium chloride, although a few units use potassium chloride instead.
The problem with replacing one molecule with a different one is that although the amount of calcium and magnesium may go down, the amount of sodium goes up by the same amount. Many plants can’t tolerate much sodium in the water, and fish that do better in soft water will have no easier time in sodium-filled water than they would in water with high levels of calcium. What is important to fish is the total dissolved solids, which doesn’t change when you exchange one molecule for another.
If you live in a hard water area and have a whole-house water softener system, make sure you install a bypass for the softener, and use unsoftened, tap water for your aquarium. Your plants will thank you, and most fish will too.
Plants need a certain amount of calcium and magnesium for proper growth. A good range for general hardness is between 50 and 150 mg/L. if your hardness is lower than 50 mg/L, you can increase it by adding calcium carbonate to your aquarium.
Source: Sunken Gardens/ By: Karen A. Randall