This comes more from intense hours of digging through cannabis forums than actual experience, but I've heard, from a very wise man on this forum named Subcool, that when cultivating Cannabis one is best to avoid slow release fertilizers. When using slow release it is impossible to know how much food your plant is receiving at any given time. This makes adding additional nutrients extremely difficult. If I was you I would stick with the fish emulsions as well as looking at getting some bone meal and some blood meal. I always try to chose organic nutrients over synthetics, but most of the time it is much easier just to buy a bottle of chemical fertilizer.