As soon as you introduce foods other than milk. If the baby is being breastfed, ensure that the mother stays well hydrated, especially during the hottest days of the year. Up to 6 months, exclusively breastfed babies do not need anything else, not even water.
If the baby is being fed with industrialized milk, ensure that the preparation of the baby is strictly followed by the indication of the amount of water and powder written in the package (do not forget that the measuring spoons must be shallow). You must ensure that you use good quality water and that it is not contaminated.
I suggest it be bottled water, preferably low mineralized (ideal for milk preparation) and balanced with pH 7. In order to avoid problems, the ideal is to vary in the brands, within those that present neutral pH (7), since its constitution in minerals is very varied. I suggest using small bottles to avoid possible contamination over time.