I don't have personal experience with growing in rockwool. I do understand they are of a high pH and need to be pre-soaked with about a 5.5pH plain water and then flushed.
Hope the below helps some. Good Luck!
Getting Started with Rockwool
1) Adjust the pH of water to 5.5. (NO LOWER).
2) If soaking A-Ok plugs, Macroplugs, or Miniblocks, mix in a little bit of plant food (no more than ¼ strength). Then just dip the starter in this pH 5.5 mix and it's ready for the seed or cutting.
3) For all other Grodan products- SOAK, FLUSH, and then PLANT! Soak in plain water (pH 5.5) for 1/2 an hour (longer if soaking granulate).
4) Flush through with pH 5.5 water mixed with some plant food. Make sure the flushed water goes to waste, then plant into your prepared Grodan block, slab, growcubes™, or granulate!
*Thanks to Grodan for use of above material on pre-soaking.