any gardening store will have seaweed or just a water soluble potassium product just ask someone there, if you still have trouble just go to discounthydro.com and order it
and about the spots on the leaves, looks to me like gnats but it is unusual that they have already made it to ur soil at such an early time. it really could be anything tho, that is a pretty shitty picture. If it were my plant i would throw it out, if its only got 1 leaf set and already getting attacked then its probly gonn die.