seedling & water questions

hi,

I was wondering if someone could help me out with some questions I currently have racking my brains,

seedlings; what is the best ph for growing seedlings in rockwool? also what is the ppm range for seedlings?

water; if using tap water is it best to leave standing for a certain amount of time before use? (maybe 24hrs) also once ph corrected and nutrients added does this newly made mix have a sell by date?

many thanks in advance,
cheers

well for the seedlings i keep my ph at a neutral 7..... as for nutes i use promix soil-less soil so i make enough nutes and water for one feeding, i dump the rest and wash pail out.. i dont like to keep premixed water/nute salution around. if using house water let it stand open for at least 24-48 hours... gets all the chlorine out.... and ppm tap water is like 400-500 ppm spring water is about 240... i opt for the spring water or do a RO setup..