How do I condition GRODAN Grow-Cubes in large bags for pH?

Make up a nutrient solution that doesn't go below pH 5.5 to soak the cubes. You can soak them right in the big bag if you plan to use all of it at once (but it will get heavy). If not, partition in smaller bags. After they have soaked a few hours, make a slit at the bottom of the bag and let it drain out in the sink.