I'm been using this and seems to be pretty good. I'll mix a small amount here and there into coco/perlite mix and also put it in the water with my feedings/watering occasionally.
https://www.amazon.com/gp/product/B00XAEABMK/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1