We recommend using clean filtered water. Reverse osmosis water or vapor distilled water are great options and have very low to zero total dissolved solids. Total dissolved solids include all organic and inorganic dissolved contaminants including metals, salts, minerals, and ions.
Filtered retail packaged bottled waters are convenient options. Remember, the water you use may have varying levels of trace elements TDS. If you are looking to remove any excess mineral additives distilled water is your best option.
We would recommend staying away from tap water as there are higher total dissolved solids including salts, calcium, chlorine, fluorides, and heavy metals. There are many choices you can make based on your lifestyle and water requirements. Chose what is appropriate for you and your baby.