I switched from rockwool to Pro Mix as my medium in my hydro system recently. (It is a top feed / return to res system) and am keeping the res PH at the same levels as I did with the rockwool (as close to 5.8 as possible) and all seems OK Had someone recently suggest PH levels should be higher as he considers it soil. I look at it as just another soilless medium I am using for my hydroponic system. I have searched many sites and find no conclusive answer to what the PH should be. (One forum poster said he contacted the company that makes Pro Mix and they said 5.8 is what they reccomend) Would appreciate input from experienced growers on this topic.