The purpose of water being added to plants is to transport nutrients throughout your house plant and give it the moisture needed for optimal plant growth. When choosing the appropriate water to use, you want to make sure it is free of chlorine and doesn't contain high levels of sodium. Rainwater, spring water, and aquarium water are most recommended for your houseplants. If you are using another type of water you may need to use fertilizer to up the nutrients required for plant growth. Always read and follow the instructions on the fertilizer you choose as too much can have a determinant affect on your plants.