People have had sucess with miracle grow but it's only when they don't add anything else to the medium. All the nutes are in there already as others have stated. I bet if you used miracle grow alone and added nothing you would have been fine.
I personally wouldn't use it because they have had a gnat infestation problem with their soils lately, it smells like chicken shit and I want to add my own nutes to whatever soil I use like most people do. (promix bx is what I have, I drove 120 miles round trip to get it). I do however know someone who is an accomplished grower who uses MG and does fine with it.
side note: Home depot may be able to order you some promix. I've seen it there before during the spring.
I personally wouldn't use it because they have had a gnat infestation problem with their soils lately, it smells like chicken shit and I want to add my own nutes to whatever soil I use like most people do. (promix bx is what I have, I drove 120 miles round trip to get it). I do however know someone who is an accomplished grower who uses MG and does fine with it.
side note: Home depot may be able to order you some promix. I've seen it there before during the spring.