you need to know what the ph is of your tap water, cause the ph will determine what nutrients are in the appropriate cation or anion state for absorption into the root, also miracle grow has alot of nitrogen in the form urea, or old piss, so you need beneficial bacteria and mychorizae of some kind to use miracle grow to its fullest.
so id say get a ph kit for like 10-15 bucks and that will solve your problem for now, and keep your eyes open for great white, 35-45 for a 3oz jar but a little bit can actually inauculate a shit ton of plants.