To prepare rockwool for use, you must saturate and condition it to lower the pH. Start by soaking the cubes or slabs overnight in nutrient solution with a pH of between 5.0 to 5.5.
To lower the pH of the nutrient solution, use dilute solutions of either calcium nitrate, nitric acid, sulfuric acid, or phosphoric acid. Which of these to choose partially depends on what stage you are at, either vegative or the flowering cycle. traditionally, use Nitric acid in the vegative stage and phosphoric acid in the flowering stage.
After soaking, the pH of the nutrient solution should be around 5.5. When using new rockwool, the pH of the nutrient solution will rise for the first few weeks, so keep a close eye on it. If the pH rises above 7, saturate the rockwool in phosphoric acid with a pH of 4.0 for 30 minutes and then flush. This will help neutralize the alkalinity and stabilize the rockwool. For best results, run the system and make adjustments until the pH is stable before you start the garden.