You could use Peter's 20-20-20, but like most fertilizers, you would have to start by using 1/4th strength of the recommended water to fertilizer ratio, because fertilizers like this easily cause fertilizer burn. But organic fertilizers in my opinion are the best, because they contain natural unprocessed nutrients. You could use 'Age Old Organics', 'Thrive Alive', bone meal, organic tea fertilizers, guano, etc.