When we first started to water our young seeds, we used regular tap water from the basement sink. However, a white substance, later identified as salt, appeared on the top of the trays and we almost lost the entire tray of seeds. What happened is that we were using water that had gone thru the water softener and contained salt. So we installed a separate water line, before the softener, that we use to water our plants.
In most big cities they use a lot of chlorine to purify the city water supply. If you use this water, it can stunt your young vegetables. So use rainwater if you can, or allow the water to sit for a day or two so the chlorine can evaporate off. To facilitate this, pour the water into a bucket rather than a milk jug, as the bucket exposes a greater portion of the water to the air and improves chlorine evaporation.
One of the cardinal rules of watering is to never use cold water from the tap to water your plants. Instead, put the water into a bucket and allow it to warm up to room temperature for a day or two (and to allow any chlorine to dissipate). Then dip your small watering can into the larger bucket to water your plants.