Distilled water is better than tap water for growing healthy plants. Tap water has additives like fluoride and chlorine that can affect a plant's growth.
A study by the University Of Saskatchewan's Department of Plant Sciences on the effects of tap water on plant growth showed that tap water exposes plants to harmful substances like salt, magnesium and calcium. Distilled water, on the other hand, is processed by boiling the water into vapor to remove all impurities and then recondensing it to be bottled. Although distilled water comes at a premium, it is much better for the plants than what comes out of a kitchen faucet.