Soft water is water that is low in calcium and magnesium ions as well as other various minerals. Soft water is naturally less abrasive and often used for washing clothes or cleaning electronic devices because it doesn't leave mineral residue. However, soft water has several disadvantages as well.
According to the University of Kentucky, soft water is harmful to certain individuals that are sensitive to heightened salinity, or sodium levels. These include diabetics or those with high blood pressure. Because water is softened using a process known as ion exchange, where mineral ions are exchanged for sodium ions, the sodium level is typically much higher in soft water.
Plants can be extraordinarily sensitive to sodium levels, and as a result, supporting plants with soft water, which is naturally high in it, can stunt their growth. On the other side of the coin, using water that's too hard (too rich in minerals) can cause blockages in the plant's root system. Distilled, neutral water is best for plants.
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Soft water is not as abrasive as hard water, making it ideal for washing clothes to prevent damage, but it is not very useful for washing skin. Soft water will have a difficult time removing soap from skin and shampoo from hair, meaning that you'll use more water when you shower.
Soft water cannot be safely used in aquariums. Fish require very strict pH levels to remain healthy in their environment, but soft water is very susceptible to pH fluctuations, which can happen with even the smallest additives like fish wastes. Those pH fluctuations could potentially harm or kill fish.