I like Dutchmaster Flowering Formula..its specifically for mj plants and is quite cheap. Any hydro ponics gardening store would probably have it. Also consider using a bud booster, which specifically feed the buds to help your buds grow in size, thc, oil production ect ect. I like Jon's Yield Booster for a bud booster, but Ive only been able to find that in a local hydroponics store. Tiger bloom is also supposed to be very effective, though I havent used it myself.
Stick with your vegging fert,(hi N), for the first 2 weeks, then switch over to your low N, hi P, flowering fert. Use the recommended dosage untill 2 weeks prior to harvest. Then plain water to flush.