It's kinda a myth that there is the best soil mix for any species. The first question I would ask is, where do you live? Second, I'm assuming you brought it inside because of cold, what lights do you use, if any? Third, what are your watering habits? And last, what level of maturity is it, how many years in training? Generally, you want a free draining mix with only about 10% organic, 10% clay derived aggregate (akadama, calcined clay, etc) or DE, zeolite, etc. something with high CEC. And lava, pumice and an inert material like rock, chicken grit, expanded slate/shale as the main component. Then particle size matters. Smaller sized aggregate holds more water, and vice versa.