Spring water is fine, it may not be neutral PH. If you're thinking about using spring water, consider using tap water that has had the chlorine evaporated out. I use gallon milk jugs filled most but not all the way, you want to leave plenty of surface area for the water to evaporate out the chlorine through.
That's what I've been using. Either, boiled water (to my preference) or water that I've had set out in the sun (top off). I just thought they both (spring and mineral) would have nutrients relevant to growing.
Spring water is neither fine nor not fine, because there is no one thing called "spring water." That label tells us the water came from a spring, that's all we know. Spring water will vary widely in terms of the minerals and other stuff that might be in it, so it's meaningless to make a blanket statement about spring water. Don't worry about trying to feed your plants with the water, use nutes for that. As long as you have decent tap water you can filter that, let it sit open 24 hours, and it should be good to go pending pH buffering.
once they have their third or fourth nodes its safe to start nutes at 1/4 -1/8 strength depending on your npk values.
You can go by number of nodes, or when the cotyledons shrivel and die, or just go by the calendar -- about 2 weeks after they first pop through the soil. And yes start at a very diluted strength, less is more.
They are stretching for light, close the distance between the plants and light. Not ready yet for feeding.
I have them on a window sill getting sunlight but its pretty chilly today so I have them inside under a weak light @[member="toastybiz"]