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.