Bumping this for you; I am fairly certain that rockwool is alkaline (has a high pH). So you'd need to bring it down, which may be achieved to some extent just with the nutrients you're using. Rockwool is non-nutritive so even seedlings need some nutrients- a solution with a ppm between 50-100. If the fertilizer doesn't do it you need pH Down, or maybe your tap water is acidic enough.