depends, are you cloning or are you seeding? If you're cloning, I add 1/4 strength nutrient solution (100ppm) included a pinch of superthrive, then I water with pH'd water after that, with every once in awhile more nutrient solution. (don't want build-up) For seeds, water alone (distilled or R.O. is what I prefer, I don't pH it as long as the rockwool is pre PH'd. The diluted nutrient solution I use later is more than sufficient to rebalance the pH)
2-3 weeks is awful long for my plants (even from seed) to go with just water, they would be pale as hell, and definatey missing some nutrients. I'd go with water through week 1, then once the true leaves have outsized the seed leaves I'd do with <100ppm. (1/8X nutrient solution)--That's just my opinion though, as after a week in rockwool I'm usually seeing roots poke through and at least a whole set of true leaves getting bigger, usually by that time 2+ sets. If you have true leaves and roots poking out the bottom they should really have some dilute nutrient solution.
